[v4,12/22] lib/stackdepot: use read/write lock

  From: Andrey Konovalov <andreyknvl@google.com>

Currently, stack depot uses the following locking scheme:

1. Lock-free accesses when looking up a stack record, which allows to
   have multiple users to look up records in parallel;
2. Spinlock for protecting the stack depot pools and the hash table
   when adding a new record.

For implementing the eviction of stack traces from stack depot, the
lock-free approach is not going to work anymore, as we will need to be
able to also remove records from the hash table.

Convert the spinlock into a read/write lock, and drop the atomic accesses,
as they are no longer required.

Looking up stack traces is now protected by the read lock and adding new
records - by the write lock. One of the following patches will add a new
function for evicting stack records, which will be protected by the write
lock as well.

With this change, multiple users can still look up records in parallel.

This is preparatory patch for implementing the eviction of stack records
from the stack depot.

Reviewed-by: Alexander Potapenko <glider@google.com>
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

---

Changed v2->v3:
- Use lockdep_assert_held_read annotation in depot_fetch_stack.

Changes v1->v2:
- Add lockdep_assert annotations.
---
 lib/stackdepot.c | 87 +++++++++++++++++++++++++-----------------------
 1 file changed, 46 insertions(+), 41 deletions(-)
  

On Mon, Nov 20, 2023 at 06:47:10PM +0100, andrey.konovalov@linux.dev wrote:
> From: Andrey Konovalov <andreyknvl@google.com>
> 
> Currently, stack depot uses the following locking scheme:
> 
> 1. Lock-free accesses when looking up a stack record, which allows to
>    have multiple users to look up records in parallel;
> 2. Spinlock for protecting the stack depot pools and the hash table
>    when adding a new record.
> 
> For implementing the eviction of stack traces from stack depot, the
> lock-free approach is not going to work anymore, as we will need to be
> able to also remove records from the hash table.
> 
> Convert the spinlock into a read/write lock, and drop the atomic accesses,
> as they are no longer required.
> 
> Looking up stack traces is now protected by the read lock and adding new
> records - by the write lock. One of the following patches will add a new
> function for evicting stack records, which will be protected by the write
> lock as well.
> 
> With this change, multiple users can still look up records in parallel.
> 
> This is preparatory patch for implementing the eviction of stack records
> from the stack depot.
> 
> Reviewed-by: Alexander Potapenko <glider@google.com>
> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>

Reviewed-by: Oscar Salvador <osalvador@suse.de>

> ---
> 
> Changed v2->v3:
> - Use lockdep_assert_held_read annotation in depot_fetch_stack.
> 
> Changes v1->v2:
> - Add lockdep_assert annotations.
> ---
>  lib/stackdepot.c | 87 +++++++++++++++++++++++++-----------------------
>  1 file changed, 46 insertions(+), 41 deletions(-)
> 
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index a5eff165c0d5..8378b32b5310 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -23,6 +23,7 @@
>  #include <linux/percpu.h>
>  #include <linux/printk.h>
>  #include <linux/slab.h>
> +#include <linux/spinlock.h>
>  #include <linux/stacktrace.h>
>  #include <linux/stackdepot.h>
>  #include <linux/string.h>
> @@ -91,15 +92,15 @@ static void *new_pool;
>  static int pools_num;
>  /* Next stack in the freelist of stack records within stack_pools. */
>  static struct stack_record *next_stack;
> -/* Lock that protects the variables above. */
> -static DEFINE_RAW_SPINLOCK(pool_lock);
>  /*
>   * Stack depot tries to keep an extra pool allocated even before it runs out
>   * of space in the currently used pool. This flag marks whether this extra pool
>   * needs to be allocated. It has the value 0 when either an extra pool is not
>   * yet allocated or if the limit on the number of pools is reached.
>   */
> -static int new_pool_required = 1;
> +static bool new_pool_required = true;
> +/* Lock that protects the variables above. */
> +static DEFINE_RWLOCK(pool_rwlock);
>  
>  static int __init disable_stack_depot(char *str)
>  {
> @@ -232,6 +233,8 @@ static void depot_init_pool(void *pool)
>  	const int records_in_pool = DEPOT_POOL_SIZE / DEPOT_STACK_RECORD_SIZE;
>  	int i, offset;
>  
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Initialize handles and link stack records to each other. */
>  	for (i = 0, offset = 0;
>  	     offset <= DEPOT_POOL_SIZE - DEPOT_STACK_RECORD_SIZE;
> @@ -254,22 +257,17 @@ static void depot_init_pool(void *pool)
>  
>  	/* Save reference to the pool to be used by depot_fetch_stack(). */
>  	stack_pools[pools_num] = pool;
> -
> -	/*
> -	 * WRITE_ONCE() pairs with potential concurrent read in
> -	 * depot_fetch_stack().
> -	 */
> -	WRITE_ONCE(pools_num, pools_num + 1);
> +	pools_num++;
>  }
>  
>  /* Keeps the preallocated memory to be used for a new stack depot pool. */
>  static void depot_keep_new_pool(void **prealloc)
>  {
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/*
>  	 * If a new pool is already saved or the maximum number of
>  	 * pools is reached, do not use the preallocated memory.
> -	 * Access new_pool_required non-atomically, as there are no concurrent
> -	 * write accesses to this variable.
>  	 */
>  	if (!new_pool_required)
>  		return;
> @@ -287,15 +285,15 @@ static void depot_keep_new_pool(void **prealloc)
>  	 * At this point, either a new pool is kept or the maximum
>  	 * number of pools is reached. In either case, take note that
>  	 * keeping another pool is not required.
> -	 * smp_store_release() pairs with smp_load_acquire() in
> -	 * stack_depot_save().
>  	 */
> -	smp_store_release(&new_pool_required, 0);
> +	new_pool_required = false;
>  }
>  
>  /* Updates references to the current and the next stack depot pools. */
>  static bool depot_update_pools(void **prealloc)
>  {
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Check if we still have objects in the freelist. */
>  	if (next_stack)
>  		goto out_keep_prealloc;
> @@ -307,7 +305,7 @@ static bool depot_update_pools(void **prealloc)
>  
>  		/* Take note that we might need a new new_pool. */
>  		if (pools_num < DEPOT_MAX_POOLS)
> -			smp_store_release(&new_pool_required, 1);
> +			new_pool_required = true;
>  
>  		/* Try keeping the preallocated memory for new_pool. */
>  		goto out_keep_prealloc;
> @@ -341,6 +339,8 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>  {
>  	struct stack_record *stack;
>  
> +	lockdep_assert_held_write(&pool_rwlock);
> +
>  	/* Update current and new pools if required and possible. */
>  	if (!depot_update_pools(prealloc))
>  		return NULL;
> @@ -376,18 +376,15 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>  static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>  {
>  	union handle_parts parts = { .handle = handle };
> -	/*
> -	 * READ_ONCE() pairs with potential concurrent write in
> -	 * depot_init_pool().
> -	 */
> -	int pools_num_cached = READ_ONCE(pools_num);
>  	void *pool;
>  	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
>  	struct stack_record *stack;
>  
> -	if (parts.pool_index > pools_num_cached) {
> +	lockdep_assert_held_read(&pool_rwlock);
> +
> +	if (parts.pool_index > pools_num) {
>  		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
> -		     parts.pool_index, pools_num_cached, handle);
> +		     parts.pool_index, pools_num, handle);
>  		return NULL;
>  	}
>  
> @@ -429,6 +426,8 @@ static inline struct stack_record *find_stack(struct stack_record *bucket,
>  {
>  	struct stack_record *found;
>  
> +	lockdep_assert_held(&pool_rwlock);
> +
>  	for (found = bucket; found; found = found->next) {
>  		if (found->hash == hash &&
>  		    found->size == size &&
> @@ -446,6 +445,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  	depot_stack_handle_t handle = 0;
>  	struct page *page = NULL;
>  	void *prealloc = NULL;
> +	bool need_alloc = false;
>  	unsigned long flags;
>  	u32 hash;
>  
> @@ -465,22 +465,26 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  	hash = hash_stack(entries, nr_entries);
>  	bucket = &stack_table[hash & stack_hash_mask];
>  
> -	/*
> -	 * Fast path: look the stack trace up without locking.
> -	 * smp_load_acquire() pairs with smp_store_release() to |bucket| below.
> -	 */
> -	found = find_stack(smp_load_acquire(bucket), entries, nr_entries, hash);
> -	if (found)
> +	read_lock_irqsave(&pool_rwlock, flags);
> +
> +	/* Fast path: look the stack trace up without full locking. */
> +	found = find_stack(*bucket, entries, nr_entries, hash);
> +	if (found) {
> +		read_unlock_irqrestore(&pool_rwlock, flags);
>  		goto exit;
> +	}
> +
> +	/* Take note if another stack pool needs to be allocated. */
> +	if (new_pool_required)
> +		need_alloc = true;
> +
> +	read_unlock_irqrestore(&pool_rwlock, flags);
>  
>  	/*
> -	 * Check if another stack pool needs to be allocated. If so, allocate
> -	 * the memory now: we won't be able to do that under the lock.
> -	 *
> -	 * smp_load_acquire() pairs with smp_store_release() in
> -	 * depot_update_pools() and depot_keep_new_pool().
> +	 * Allocate memory for a new pool if required now:
> +	 * we won't be able to do that under the lock.
>  	 */
> -	if (unlikely(can_alloc && smp_load_acquire(&new_pool_required))) {
> +	if (unlikely(can_alloc && need_alloc)) {
>  		/*
>  		 * Zero out zone modifiers, as we don't have specific zone
>  		 * requirements. Keep the flags related to allocation in atomic
> @@ -494,7 +498,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  			prealloc = page_address(page);
>  	}
>  
> -	raw_spin_lock_irqsave(&pool_lock, flags);
> +	write_lock_irqsave(&pool_rwlock, flags);
>  
>  	found = find_stack(*bucket, entries, nr_entries, hash);
>  	if (!found) {
> @@ -503,11 +507,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  
>  		if (new) {
>  			new->next = *bucket;
> -			/*
> -			 * smp_store_release() pairs with smp_load_acquire()
> -			 * from |bucket| above.
> -			 */
> -			smp_store_release(bucket, new);
> +			*bucket = new;
>  			found = new;
>  		}
>  	} else if (prealloc) {
> @@ -518,7 +518,7 @@ depot_stack_handle_t __stack_depot_save(unsigned long *entries,
>  		depot_keep_new_pool(&prealloc);
>  	}
>  
> -	raw_spin_unlock_irqrestore(&pool_lock, flags);
> +	write_unlock_irqrestore(&pool_rwlock, flags);
>  exit:
>  	if (prealloc) {
>  		/* Stack depot didn't use this memory, free it. */
> @@ -542,6 +542,7 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>  			       unsigned long **entries)
>  {
>  	struct stack_record *stack;
> +	unsigned long flags;
>  
>  	*entries = NULL;
>  	/*
> @@ -553,8 +554,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>  	if (!handle || stack_depot_disabled)
>  		return 0;
>  
> +	read_lock_irqsave(&pool_rwlock, flags);
> +
>  	stack = depot_fetch_stack(handle);
>  
> +	read_unlock_irqrestore(&pool_rwlock, flags);
> +
>  	*entries = stack->entries;
>  	return stack->size;
>  }
> -- 
> 2.25.1
>
  

Oscar Salvador <osalvador@suse.de> writes:
>> 
>> With this change, multiple users can still look up records in parallel.

That's a severe misunderstanding -- rwlocks always bounce a cache line,
so the parallelism is significantly reduced.

Normally rwlocks are only worth it if your critical region is quite long.

>> 
>> This is preparatory patch for implementing the eviction of stack records
>> from the stack depot.
>> 
>> Reviewed-by: Alexander Potapenko <glider@google.com>
>> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
>
> Reviewed-by: Oscar Salvador <osalvador@suse.de>


Has anyone benchmarked this on a high core count machine? It sounds
pretty bad if every lock aquisition starts bouncing a single cache line.

Consider using RCU or similar.

-Andi
  

On Thu, 11 Jan 2024 at 00:01, Andi Kleen <ak@linux.intel.com> wrote:
>
> Oscar Salvador <osalvador@suse.de> writes:
> >>
> >> With this change, multiple users can still look up records in parallel.
>
> That's a severe misunderstanding -- rwlocks always bounce a cache line,
> so the parallelism is significantly reduced.
>
> Normally rwlocks are only worth it if your critical region is quite long.
>
> >>
> >> This is preparatory patch for implementing the eviction of stack records
> >> from the stack depot.
> >>
> >> Reviewed-by: Alexander Potapenko <glider@google.com>
> >> Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
> >
> > Reviewed-by: Oscar Salvador <osalvador@suse.de>
>
>
> Has anyone benchmarked this on a high core count machine? It sounds
> pretty bad if every lock aquisition starts bouncing a single cache line.
>
> Consider using RCU or similar.

stackdepot is severely limited in what kernel facilities it may use
due to being used by such low level facilities as the allocator
itself.

I've been suggesting percpu-rwsem here, but looking at it in more
detail that doesn't work because percpu-rwsem wants to sleep, but
stackdepot must work in non-sleepable contexts. :-/
  

> stackdepot is severely limited in what kernel facilities it may use
> due to being used by such low level facilities as the allocator
> itself.

RCU can be done quite low level too (e.g. there is NMI safe RCU)

> 
> I've been suggesting percpu-rwsem here, but looking at it in more
> detail that doesn't work because percpu-rwsem wants to sleep, but
> stackdepot must work in non-sleepable contexts. :-/

Yes something per CPU would work too I suppose. We used to have
big reader spinlocks for this. 

-Andi
  

On Thu, Jan 11, 2024 at 04:36AM -0800, Andi Kleen wrote:
> > stackdepot is severely limited in what kernel facilities it may use
> > due to being used by such low level facilities as the allocator
> > itself.
> 
> RCU can be done quite low level too (e.g. there is NMI safe RCU)

How about the below? This should get us back the performance of the old
lock-less version. Although it's using rculist, we don't actually need
to synchronize via RCU.

Thanks,
-- Marco

------ >8 ------

From: Marco Elver <elver@google.com>
Date: Tue, 9 Jan 2024 10:21:56 +0100
Subject: [PATCH] stackdepot: make fast paths lock-less again

stack_depot_put() unconditionally takes the pool_rwlock as a writer.
This is unnecessary if the stack record is not going to be freed.
Furthermore, reader-writer locks have inherent cache contention, which
does not scale well on machines with large CPU counts.

Instead, rework the synchronization story of stack depot to again avoid
taking any locks in the fast paths. This is done by relying on RCU
primitives to give us lock-less list traversal. See code comments for
more details.

Fixes: 108be8def46e ("lib/stackdepot: allow users to evict stack traces")
Signed-off-by: Marco Elver <elver@google.com>
---
 lib/stackdepot.c | 222 ++++++++++++++++++++++++++++-------------------
 1 file changed, 133 insertions(+), 89 deletions(-)

diff --git a/lib/stackdepot.c b/lib/stackdepot.c
index a0be5d05c7f0..9eaf46f8abc4 100644
--- a/lib/stackdepot.c
+++ b/lib/stackdepot.c
@@ -19,10 +19,13 @@
 #include <linux/kernel.h>
 #include <linux/kmsan.h>
 #include <linux/list.h>
+#include <linux/llist.h>
 #include <linux/mm.h>
 #include <linux/mutex.h>
 #include <linux/percpu.h>
 #include <linux/printk.h>
+#include <linux/rculist.h>
+#include <linux/rcupdate.h>
 #include <linux/refcount.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
@@ -67,7 +70,8 @@ union handle_parts {
 };
 
 struct stack_record {
-	struct list_head list;		/* Links in hash table or freelist */
+	struct list_head hash_list;	/* Links in the hash table */
+	struct llist_node free_list;	/* Links in the freelist */
 	u32 hash;			/* Hash in hash table */
 	u32 size;			/* Number of stored frames */
 	union handle_parts handle;
@@ -104,7 +108,7 @@ static void *new_pool;
 /* Number of pools in stack_pools. */
 static int pools_num;
 /* Freelist of stack records within stack_pools. */
-static LIST_HEAD(free_stacks);
+static LLIST_HEAD(free_stacks);
 /*
  * Stack depot tries to keep an extra pool allocated even before it runs out
  * of space in the currently used pool. This flag marks whether this extra pool
@@ -112,8 +116,8 @@ static LIST_HEAD(free_stacks);
  * yet allocated or if the limit on the number of pools is reached.
  */
 static bool new_pool_required = true;
-/* Lock that protects the variables above. */
-static DEFINE_RWLOCK(pool_rwlock);
+/* The lock must be held when performing pool or free list modifications. */
+static DEFINE_RAW_SPINLOCK(pool_lock);
 
 static int __init disable_stack_depot(char *str)
 {
@@ -263,9 +267,7 @@ static void depot_init_pool(void *pool)
 {
 	int offset;
 
-	lockdep_assert_held_write(&pool_rwlock);
-
-	WARN_ON(!list_empty(&free_stacks));
+	lockdep_assert_held(&pool_lock);
 
 	/* Initialize handles and link stack records into the freelist. */
 	for (offset = 0; offset <= DEPOT_POOL_SIZE - DEPOT_STACK_RECORD_SIZE;
@@ -276,18 +278,25 @@ static void depot_init_pool(void *pool)
 		stack->handle.offset = offset >> DEPOT_STACK_ALIGN;
 		stack->handle.extra = 0;
 
-		list_add(&stack->list, &free_stacks);
+		llist_add(&stack->free_list, &free_stacks);
+		INIT_LIST_HEAD(&stack->hash_list);
 	}
 
 	/* Save reference to the pool to be used by depot_fetch_stack(). */
 	stack_pools[pools_num] = pool;
-	pools_num++;
+
+	/*
+	 * Release of pool pointer assignment above. Pairs with the
+	 * smp_load_acquire() in depot_fetch_stack().
+	 */
+	smp_store_release(&pools_num, pools_num + 1);
+	ASSERT_EXCLUSIVE_WRITER(pools_num);
 }
 
 /* Keeps the preallocated memory to be used for a new stack depot pool. */
 static void depot_keep_new_pool(void **prealloc)
 {
-	lockdep_assert_held_write(&pool_rwlock);
+	lockdep_assert_held(&pool_lock);
 
 	/*
 	 * If a new pool is already saved or the maximum number of
@@ -310,16 +319,16 @@ static void depot_keep_new_pool(void **prealloc)
 	 * number of pools is reached. In either case, take note that
 	 * keeping another pool is not required.
 	 */
-	new_pool_required = false;
+	WRITE_ONCE(new_pool_required, false);
 }
 
 /* Updates references to the current and the next stack depot pools. */
 static bool depot_update_pools(void **prealloc)
 {
-	lockdep_assert_held_write(&pool_rwlock);
+	lockdep_assert_held(&pool_lock);
 
 	/* Check if we still have objects in the freelist. */
-	if (!list_empty(&free_stacks))
+	if (!llist_empty(&free_stacks))
 		goto out_keep_prealloc;
 
 	/* Check if we have a new pool saved and use it. */
@@ -329,7 +338,7 @@ static bool depot_update_pools(void **prealloc)
 
 		/* Take note that we might need a new new_pool. */
 		if (pools_num < DEPOT_MAX_POOLS)
-			new_pool_required = true;
+			WRITE_ONCE(new_pool_required, true);
 
 		/* Try keeping the preallocated memory for new_pool. */
 		goto out_keep_prealloc;
@@ -362,20 +371,19 @@ static struct stack_record *
 depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 {
 	struct stack_record *stack;
+	struct llist_node *free;
 
-	lockdep_assert_held_write(&pool_rwlock);
+	lockdep_assert_held(&pool_lock);
 
 	/* Update current and new pools if required and possible. */
 	if (!depot_update_pools(prealloc))
 		return NULL;
 
 	/* Check if we have a stack record to save the stack trace. */
-	if (list_empty(&free_stacks))
+	free = llist_del_first(&free_stacks);
+	if (!free)
 		return NULL;
-
-	/* Get and unlink the first entry from the freelist. */
-	stack = list_first_entry(&free_stacks, struct stack_record, list);
-	list_del(&stack->list);
+	stack = llist_entry(free, struct stack_record, free_list);
 
 	/* Limit number of saved frames to CONFIG_STACKDEPOT_MAX_FRAMES. */
 	if (size > CONFIG_STACKDEPOT_MAX_FRAMES)
@@ -385,7 +393,6 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	stack->hash = hash;
 	stack->size = size;
 	/* stack->handle is already filled in by depot_init_pool(). */
-	refcount_set(&stack->count, 1);
 	memcpy(stack->entries, entries, flex_array_size(stack, entries, size));
 
 	/*
@@ -394,21 +401,30 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
 	 */
 	kmsan_unpoison_memory(stack, DEPOT_STACK_RECORD_SIZE);
 
+	/*
+	 * Release saving of the stack trace. Pairs with smp_mb() in
+	 * depot_fetch_stack().
+	 */
+	smp_mb__before_atomic();
+	refcount_set(&stack->count, 1);
+
 	return stack;
 }
 
 static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
 {
+	/* Acquire the pool pointer written in depot_init_pool(). */
+	const int pools_num_cached = smp_load_acquire(&pools_num);
 	union handle_parts parts = { .handle = handle };
 	void *pool;
 	size_t offset = parts.offset << DEPOT_STACK_ALIGN;
 	struct stack_record *stack;
 
-	lockdep_assert_held(&pool_rwlock);
+	lockdep_assert_not_held(&pool_lock);
 
-	if (parts.pool_index > pools_num) {
+	if (parts.pool_index > pools_num_cached) {
 		WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
-		     parts.pool_index, pools_num, handle);
+		     parts.pool_index, pools_num_cached, handle);
 		return NULL;
 	}
 
@@ -417,15 +433,35 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
 		return NULL;
 
 	stack = pool + offset;
+
+	/*
+	 * Acquire the stack trace. Pairs with smp_mb() in depot_alloc_stack().
+	 *
+	 * This does not protect against a stack_depot_put() freeing the record
+	 * and having it subsequently being reused. Callers are responsible to
+	 * avoid using stack depot handles after passing to stack_depot_put().
+	 */
+	if (!refcount_read(&stack->count))
+		return NULL;
+	smp_mb__after_atomic();
+
 	return stack;
 }
 
 /* Links stack into the freelist. */
 static void depot_free_stack(struct stack_record *stack)
 {
-	lockdep_assert_held_write(&pool_rwlock);
+	unsigned long flags;
+
+	lockdep_assert_not_held(&pool_lock);
+
+	raw_spin_lock_irqsave(&pool_lock, flags);
+	printk_deferred_enter();
+	list_del_rcu(&stack->hash_list);
+	printk_deferred_exit();
+	raw_spin_unlock_irqrestore(&pool_lock, flags);
 
-	list_add(&stack->list, &free_stacks);
+	llist_add(&stack->free_list, &free_stacks);
 }
 
 /* Calculates the hash for a stack. */
@@ -453,22 +489,55 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
 
 /* Finds a stack in a bucket of the hash table. */
 static inline struct stack_record *find_stack(struct list_head *bucket,
-					     unsigned long *entries, int size,
-					     u32 hash)
+					      unsigned long *entries, int size,
+					      u32 hash, depot_flags_t flags)
 {
-	struct list_head *pos;
-	struct stack_record *found;
+	struct stack_record *stack, *ret = NULL;
 
-	lockdep_assert_held(&pool_rwlock);
+	/*
+	 * Due to being used from low-level code paths such as the allocators,
+	 * NMI, or even RCU itself, stackdepot cannot rely on primitives that
+	 * would sleep (such as synchronize_rcu()) or end up recursively call
+	 * into stack depot again (such as call_rcu()).
+	 *
+	 * Instead, lock-less readers only rely on RCU primitives for correct
+	 * memory ordering, but do not use RCU-based synchronization otherwise.
+	 * Instead, we perform 3-pass validation below to ensure that the stack
+	 * record we accessed is actually valid. If we fail to obtain a valid
+	 * stack record here, the slow-path in stack_depot_save_flags() will
+	 * retry to avoid inserting duplicates.
+	 *
+	 * If STACK_DEPOT_FLAG_GET is not used, it is undefined behaviour to
+	 * call stack_depot_put() later - i.e. in the non-refcounted case, we do
+	 * not have to worry that the entry will be recycled.
+	 */
+
+	list_for_each_entry_rcu(stack, bucket, hash_list) {
+		/* 1. Check if this entry could potentially match. */
+		if (data_race(stack->hash != hash || stack->size != size))
+			continue;
+
+		/*
+		 * 2. Increase refcount if not zero. If this is successful, we
+		 *    know that this stack record is valid and will not be freed by
+		 *    stack_depot_put().
+		 */
+		if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(!refcount_inc_not_zero(&stack->count)))
+			continue;
+
+		/* 3. Do full validation of the record. */
+		if (likely(stack->hash == hash && stack->size == size &&
+			   !stackdepot_memcmp(entries, stack->entries, size))) {
+			ret = stack;
+			break;
+		}
 
-	list_for_each(pos, bucket) {
-		found = list_entry(pos, struct stack_record, list);
-		if (found->hash == hash &&
-		    found->size == size &&
-		    !stackdepot_memcmp(entries, found->entries, size))
-			return found;
+		/* Undo refcount - could have raced with stack_depot_put(). */
+		if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(refcount_dec_and_test(&stack->count)))
+			depot_free_stack(stack);
 	}
-	return NULL;
+
+	return ret;
 }
 
 depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
@@ -482,7 +551,6 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
 	struct page *page = NULL;
 	void *prealloc = NULL;
 	bool can_alloc = depot_flags & STACK_DEPOT_FLAG_CAN_ALLOC;
-	bool need_alloc = false;
 	unsigned long flags;
 	u32 hash;
 
@@ -505,31 +573,16 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
 	hash = hash_stack(entries, nr_entries);
 	bucket = &stack_table[hash & stack_hash_mask];
 
-	read_lock_irqsave(&pool_rwlock, flags);
-	printk_deferred_enter();
-
-	/* Fast path: look the stack trace up without full locking. */
-	found = find_stack(bucket, entries, nr_entries, hash);
-	if (found) {
-		if (depot_flags & STACK_DEPOT_FLAG_GET)
-			refcount_inc(&found->count);
-		printk_deferred_exit();
-		read_unlock_irqrestore(&pool_rwlock, flags);
+	/* Fast path: look the stack trace up without locking. */
+	found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
+	if (found)
 		goto exit;
-	}
-
-	/* Take note if another stack pool needs to be allocated. */
-	if (new_pool_required)
-		need_alloc = true;
-
-	printk_deferred_exit();
-	read_unlock_irqrestore(&pool_rwlock, flags);
 
 	/*
 	 * Allocate memory for a new pool if required now:
 	 * we won't be able to do that under the lock.
 	 */
-	if (unlikely(can_alloc && need_alloc)) {
+	if (unlikely(can_alloc && READ_ONCE(new_pool_required))) {
 		/*
 		 * Zero out zone modifiers, as we don't have specific zone
 		 * requirements. Keep the flags related to allocation in atomic
@@ -543,31 +596,33 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
 			prealloc = page_address(page);
 	}
 
-	write_lock_irqsave(&pool_rwlock, flags);
+	raw_spin_lock_irqsave(&pool_lock, flags);
 	printk_deferred_enter();
 
-	found = find_stack(bucket, entries, nr_entries, hash);
+	/* Try to find again, to avoid concurrently inserting duplicates. */
+	found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
 	if (!found) {
 		struct stack_record *new =
 			depot_alloc_stack(entries, nr_entries, hash, &prealloc);
 
 		if (new) {
-			list_add(&new->list, bucket);
+			/*
+			 * This releases the stack record into the bucket and
+			 * makes it visible to readers in find_stack().
+			 */
+			list_add_rcu(&new->hash_list, bucket);
 			found = new;
 		}
-	} else {
-		if (depot_flags & STACK_DEPOT_FLAG_GET)
-			refcount_inc(&found->count);
+	} else if (prealloc) {
 		/*
 		 * Stack depot already contains this stack trace, but let's
 		 * keep the preallocated memory for future.
 		 */
-		if (prealloc)
-			depot_keep_new_pool(&prealloc);
+		depot_keep_new_pool(&prealloc);
 	}
 
 	printk_deferred_exit();
-	write_unlock_irqrestore(&pool_rwlock, flags);
+	raw_spin_unlock_irqrestore(&pool_lock, flags);
 exit:
 	if (prealloc) {
 		/* Stack depot didn't use this memory, free it. */
@@ -592,7 +647,6 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 			       unsigned long **entries)
 {
 	struct stack_record *stack;
-	unsigned long flags;
 
 	*entries = NULL;
 	/*
@@ -604,13 +658,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
 	if (!handle || stack_depot_disabled)
 		return 0;
 
-	read_lock_irqsave(&pool_rwlock, flags);
-	printk_deferred_enter();
-
 	stack = depot_fetch_stack(handle);
-
-	printk_deferred_exit();
-	read_unlock_irqrestore(&pool_rwlock, flags);
+	/*
+	 * Should never be NULL, otherwise this is a use-after-put.
+	 */
+	if (WARN_ON(!stack))
+		return 0;
 
 	*entries = stack->entries;
 	return stack->size;
@@ -620,29 +673,20 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
 void stack_depot_put(depot_stack_handle_t handle)
 {
 	struct stack_record *stack;
-	unsigned long flags;
 
 	if (!handle || stack_depot_disabled)
 		return;
 
-	write_lock_irqsave(&pool_rwlock, flags);
-	printk_deferred_enter();
-
 	stack = depot_fetch_stack(handle);
+	/*
+	 * Should always be able to find the stack record, otherwise this is an
+	 * unbalanced put attempt.
+	 */
 	if (WARN_ON(!stack))
-		goto out;
-
-	if (refcount_dec_and_test(&stack->count)) {
-		/* Unlink stack from the hash table. */
-		list_del(&stack->list);
+		return;
 
-		/* Free stack. */
+	if (refcount_dec_and_test(&stack->count))
 		depot_free_stack(stack);
-	}
-
-out:
-	printk_deferred_exit();
-	write_unlock_irqrestore(&pool_rwlock, flags);
 }
 EXPORT_SYMBOL_GPL(stack_depot_put);
  

On Thu, Jan 11, 2024 at 8:08 PM Marco Elver <elver@google.com> wrote:
>
> On Thu, Jan 11, 2024 at 04:36AM -0800, Andi Kleen wrote:
> > > stackdepot is severely limited in what kernel facilities it may use
> > > due to being used by such low level facilities as the allocator
> > > itself.
> >
> > RCU can be done quite low level too (e.g. there is NMI safe RCU)
>
> How about the below? This should get us back the performance of the old
> lock-less version. Although it's using rculist, we don't actually need
> to synchronize via RCU.
>
> Thanks,
> -- Marco
>
> ------ >8 ------
>
> From: Marco Elver <elver@google.com>
> Date: Tue, 9 Jan 2024 10:21:56 +0100
> Subject: [PATCH] stackdepot: make fast paths lock-less again
>
> stack_depot_put() unconditionally takes the pool_rwlock as a writer.
> This is unnecessary if the stack record is not going to be freed.
> Furthermore, reader-writer locks have inherent cache contention, which
> does not scale well on machines with large CPU counts.
>
> Instead, rework the synchronization story of stack depot to again avoid
> taking any locks in the fast paths. This is done by relying on RCU
> primitives to give us lock-less list traversal. See code comments for
> more details.
>
> Fixes: 108be8def46e ("lib/stackdepot: allow users to evict stack traces")
> Signed-off-by: Marco Elver <elver@google.com>
> ---
>  lib/stackdepot.c | 222 ++++++++++++++++++++++++++++-------------------
>  1 file changed, 133 insertions(+), 89 deletions(-)
>
> diff --git a/lib/stackdepot.c b/lib/stackdepot.c
> index a0be5d05c7f0..9eaf46f8abc4 100644
> --- a/lib/stackdepot.c
> +++ b/lib/stackdepot.c
> @@ -19,10 +19,13 @@
>  #include <linux/kernel.h>
>  #include <linux/kmsan.h>
>  #include <linux/list.h>
> +#include <linux/llist.h>
>  #include <linux/mm.h>
>  #include <linux/mutex.h>
>  #include <linux/percpu.h>
>  #include <linux/printk.h>
> +#include <linux/rculist.h>
> +#include <linux/rcupdate.h>
>  #include <linux/refcount.h>
>  #include <linux/slab.h>
>  #include <linux/spinlock.h>
> @@ -67,7 +70,8 @@ union handle_parts {
>  };
>
>  struct stack_record {
> -       struct list_head list;          /* Links in hash table or freelist */
> +       struct list_head hash_list;     /* Links in the hash table */
> +       struct llist_node free_list;    /* Links in the freelist */
>         u32 hash;                       /* Hash in hash table */
>         u32 size;                       /* Number of stored frames */
>         union handle_parts handle;
> @@ -104,7 +108,7 @@ static void *new_pool;
>  /* Number of pools in stack_pools. */
>  static int pools_num;
>  /* Freelist of stack records within stack_pools. */
> -static LIST_HEAD(free_stacks);
> +static LLIST_HEAD(free_stacks);
>  /*
>   * Stack depot tries to keep an extra pool allocated even before it runs out
>   * of space in the currently used pool. This flag marks whether this extra pool
> @@ -112,8 +116,8 @@ static LIST_HEAD(free_stacks);
>   * yet allocated or if the limit on the number of pools is reached.
>   */
>  static bool new_pool_required = true;
> -/* Lock that protects the variables above. */
> -static DEFINE_RWLOCK(pool_rwlock);
> +/* The lock must be held when performing pool or free list modifications */
> +static DEFINE_RAW_SPINLOCK(pool_lock);
>
>  static int __init disable_stack_depot(char *str)
>  {
> @@ -263,9 +267,7 @@ static void depot_init_pool(void *pool)
>  {
>         int offset;
>
> -       lockdep_assert_held_write(&pool_rwlock);
> -
> -       WARN_ON(!list_empty(&free_stacks));
> +       lockdep_assert_held(&pool_lock);
>
>         /* Initialize handles and link stack records into the freelist. */
>         for (offset = 0; offset <= DEPOT_POOL_SIZE - DEPOT_STACK_RECORD_SIZE;
> @@ -276,18 +278,25 @@ static void depot_init_pool(void *pool)
>                 stack->handle.offset = offset >> DEPOT_STACK_ALIGN;
>                 stack->handle.extra = 0;
>
> -               list_add(&stack->list, &free_stacks);
> +               llist_add(&stack->free_list, &free_stacks);
> +               INIT_LIST_HEAD(&stack->hash_list);
>         }
>
>         /* Save reference to the pool to be used by depot_fetch_stack(). */
>         stack_pools[pools_num] = pool;
> -       pools_num++;
> +
> +       /*
> +        * Release of pool pointer assignment above. Pairs with the
> +        * smp_load_acquire() in depot_fetch_stack().
> +        */
> +       smp_store_release(&pools_num, pools_num + 1);
> +       ASSERT_EXCLUSIVE_WRITER(pools_num);
>  }
>
>  /* Keeps the preallocated memory to be used for a new stack depot pool. */
>  static void depot_keep_new_pool(void **prealloc)
>  {
> -       lockdep_assert_held_write(&pool_rwlock);
> +       lockdep_assert_held(&pool_lock);
>
>         /*
>          * If a new pool is already saved or the maximum number of
> @@ -310,16 +319,16 @@ static void depot_keep_new_pool(void **prealloc)
>          * number of pools is reached. In either case, take note that
>          * keeping another pool is not required.
>          */
> -       new_pool_required = false;
> +       WRITE_ONCE(new_pool_required, false);
>  }
>
>  /* Updates references to the current and the next stack depot pools. */
>  static bool depot_update_pools(void **prealloc)
>  {
> -       lockdep_assert_held_write(&pool_rwlock);
> +       lockdep_assert_held(&pool_lock);
>
>         /* Check if we still have objects in the freelist. */
> -       if (!list_empty(&free_stacks))
> +       if (!llist_empty(&free_stacks))
>                 goto out_keep_prealloc;
>
>         /* Check if we have a new pool saved and use it. */
> @@ -329,7 +338,7 @@ static bool depot_update_pools(void **prealloc)
>
>                 /* Take note that we might need a new new_pool. */
>                 if (pools_num < DEPOT_MAX_POOLS)
> -                       new_pool_required = true;
> +                       WRITE_ONCE(new_pool_required, true);
>
>                 /* Try keeping the preallocated memory for new_pool. */
>                 goto out_keep_prealloc;
> @@ -362,20 +371,19 @@ static struct stack_record *
>  depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>  {
>         struct stack_record *stack;
> +       struct llist_node *free;
>
> -       lockdep_assert_held_write(&pool_rwlock);
> +       lockdep_assert_held(&pool_lock);
>
>         /* Update current and new pools if required and possible. */
>         if (!depot_update_pools(prealloc))
>                 return NULL;
>
>         /* Check if we have a stack record to save the stack trace. */
> -       if (list_empty(&free_stacks))
> +       free = llist_del_first(&free_stacks);
> +       if (!free)
>                 return NULL;
> -
> -       /* Get and unlink the first entry from the freelist. */
> -       stack = list_first_entry(&free_stacks, struct stack_record, list);
> -       list_del(&stack->list);
> +       stack = llist_entry(free, struct stack_record, free_list);
>
>         /* Limit number of saved frames to CONFIG_STACKDEPOT_MAX_FRAMES. */
>         if (size > CONFIG_STACKDEPOT_MAX_FRAMES)
> @@ -385,7 +393,6 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>         stack->hash = hash;
>         stack->size = size;
>         /* stack->handle is already filled in by depot_init_pool(). */
> -       refcount_set(&stack->count, 1);
>         memcpy(stack->entries, entries, flex_array_size(stack, entries, size));
>
>         /*
> @@ -394,21 +401,30 @@ depot_alloc_stack(unsigned long *entries, int size, u32 hash, void **prealloc)
>          */
>         kmsan_unpoison_memory(stack, DEPOT_STACK_RECORD_SIZE);
>
> +       /*
> +        * Release saving of the stack trace. Pairs with smp_mb() in
> +        * depot_fetch_stack().
> +        */
> +       smp_mb__before_atomic();
> +       refcount_set(&stack->count, 1);
> +
>         return stack;
>  }
>
>  static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>  {
> +       /* Acquire the pool pointer written in depot_init_pool(). */
> +       const int pools_num_cached = smp_load_acquire(&pools_num);
>         union handle_parts parts = { .handle = handle };
>         void *pool;
>         size_t offset = parts.offset << DEPOT_STACK_ALIGN;
>         struct stack_record *stack;
>
> -       lockdep_assert_held(&pool_rwlock);
> +       lockdep_assert_not_held(&pool_lock);
>
> -       if (parts.pool_index > pools_num) {
> +       if (parts.pool_index > pools_num_cached) {
>                 WARN(1, "pool index %d out of bounds (%d) for stack id %08x\n",
> -                    parts.pool_index, pools_num, handle);
> +                    parts.pool_index, pools_num_cached, handle);
>                 return NULL;
>         }
>
> @@ -417,15 +433,35 @@ static struct stack_record *depot_fetch_stack(depot_stack_handle_t handle)
>                 return NULL;
>
>         stack = pool + offset;
> +
> +       /*
> +        * Acquire the stack trace. Pairs with smp_mb() in depot_alloc_stack().
> +        *
> +        * This does not protect against a stack_depot_put() freeing the record
> +        * and having it subsequently being reused. Callers are responsible to
> +        * avoid using stack depot handles after passing to stack_depot_put().
> +        */
> +       if (!refcount_read(&stack->count))
> +               return NULL;

Can this happen? It seems that depot_fetch_stack should only be called
for handles that were returned from stack_depot_save_flags before all
puts and thus the the refcount should > 0. Or is this a safeguard
against improper API usage?

> +       smp_mb__after_atomic();
> +
>         return stack;
>  }
>
>  /* Links stack into the freelist. */
>  static void depot_free_stack(struct stack_record *stack)
>  {
> -       lockdep_assert_held_write(&pool_rwlock);
> +       unsigned long flags;
> +
> +       lockdep_assert_not_held(&pool_lock);
> +
> +       raw_spin_lock_irqsave(&pool_lock, flags);
> +       printk_deferred_enter();
> +       list_del_rcu(&stack->hash_list);
> +       printk_deferred_exit();
> +       raw_spin_unlock_irqrestore(&pool_lock, flags);
>
> -       list_add(&stack->list, &free_stacks);
> +       llist_add(&stack->free_list, &free_stacks);

This llist_add is outside of the lock just because we can (i.e.
llist_add can run concurrently with the other free_stacks operations,
which are all under the lock), right? This slightly contradicts the
comment above the free_stacks definition.

If we put this under the lock and use normal list instead of llist, I
think we can then combine the hash_list with the free_list like before
to save up on some space for stack_record. Would that make sense?

>  }
>
>  /* Calculates the hash for a stack. */
> @@ -453,22 +489,55 @@ int stackdepot_memcmp(const unsigned long *u1, const unsigned long *u2,
>
>  /* Finds a stack in a bucket of the hash table. */
>  static inline struct stack_record *find_stack(struct list_head *bucket,
> -                                            unsigned long *entries, int size,
> -                                            u32 hash)
> +                                             unsigned long *entries, int size,
> +                                             u32 hash, depot_flags_t flags)
>  {
> -       struct list_head *pos;
> -       struct stack_record *found;
> +       struct stack_record *stack, *ret = NULL;
>
> -       lockdep_assert_held(&pool_rwlock);
> +       /*
> +        * Due to being used from low-level code paths such as the allocators,
> +        * NMI, or even RCU itself, stackdepot cannot rely on primitives that
> +        * would sleep (such as synchronize_rcu()) or end up recursively call
> +        * into stack depot again (such as call_rcu()).
> +        *
> +        * Instead, lock-less readers only rely on RCU primitives for correct
> +        * memory ordering, but do not use RCU-based synchronization otherwise.
> +        * Instead, we perform 3-pass validation below to ensure that the stack
> +        * record we accessed is actually valid. If we fail to obtain a valid
> +        * stack record here, the slow-path in stack_depot_save_flags() will
> +        * retry to avoid inserting duplicates.
> +        *
> +        * If STACK_DEPOT_FLAG_GET is not used, it is undefined behaviour to
> +        * call stack_depot_put() later - i.e. in the non-refcounted case, we do
> +        * not have to worry that the entry will be recycled.
> +        */
> +
> +       list_for_each_entry_rcu(stack, bucket, hash_list) {

So we don't need rcu_read_lock here, because we don't rely on call_rcu
etc., right?

> +               /* 1. Check if this entry could potentially match. */
> +               if (data_race(stack->hash != hash || stack->size != size))
> +                       continue;
> +
> +               /*
> +                * 2. Increase refcount if not zero. If this is successful, we
> +                *    know that this stack record is valid and will not be freed by
> +                *    stack_depot_put().
> +                */
> +               if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(!refcount_inc_not_zero(&stack->count)))
> +                       continue;
> +
> +               /* 3. Do full validation of the record. */
> +               if (likely(stack->hash == hash && stack->size == size &&
> +                          !stackdepot_memcmp(entries, stack->entries, size))) {
> +                       ret = stack;
> +                       break;
> +               }
>
> -       list_for_each(pos, bucket) {
> -               found = list_entry(pos, struct stack_record, list);
> -               if (found->hash == hash &&
> -                   found->size == size &&
> -                   !stackdepot_memcmp(entries, found->entries, size))
> -                       return found;
> +               /* Undo refcount - could have raced with stack_depot_put(). */
> +               if ((flags & STACK_DEPOT_FLAG_GET) && unlikely(refcount_dec_and_test(&stack->count)))
> +                       depot_free_stack(stack);
>         }
> -       return NULL;
> +
> +       return ret;
>  }
>
>  depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
> @@ -482,7 +551,6 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
>         struct page *page = NULL;
>         void *prealloc = NULL;
>         bool can_alloc = depot_flags & STACK_DEPOT_FLAG_CAN_ALLOC;
> -       bool need_alloc = false;
>         unsigned long flags;
>         u32 hash;
>
> @@ -505,31 +573,16 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
>         hash = hash_stack(entries, nr_entries);
>         bucket = &stack_table[hash & stack_hash_mask];
>
> -       read_lock_irqsave(&pool_rwlock, flags);
> -       printk_deferred_enter();
> -
> -       /* Fast path: look the stack trace up without full locking. */
> -       found = find_stack(bucket, entries, nr_entries, hash);
> -       if (found) {
> -               if (depot_flags & STACK_DEPOT_FLAG_GET)
> -                       refcount_inc(&found->count);
> -               printk_deferred_exit();
> -               read_unlock_irqrestore(&pool_rwlock, flags);
> +       /* Fast path: look the stack trace up without locking. */
> +       found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
> +       if (found)
>                 goto exit;
> -       }
> -
> -       /* Take note if another stack pool needs to be allocated. */
> -       if (new_pool_required)
> -               need_alloc = true;
> -
> -       printk_deferred_exit();
> -       read_unlock_irqrestore(&pool_rwlock, flags);
>
>         /*
>          * Allocate memory for a new pool if required now:
>          * we won't be able to do that under the lock.
>          */
> -       if (unlikely(can_alloc && need_alloc)) {
> +       if (unlikely(can_alloc && READ_ONCE(new_pool_required))) {
>                 /*
>                  * Zero out zone modifiers, as we don't have specific zone
>                  * requirements. Keep the flags related to allocation in atomic
> @@ -543,31 +596,33 @@ depot_stack_handle_t stack_depot_save_flags(unsigned long *entries,
>                         prealloc = page_address(page);
>         }
>
> -       write_lock_irqsave(&pool_rwlock, flags);
> +       raw_spin_lock_irqsave(&pool_lock, flags);
>         printk_deferred_enter();
>
> -       found = find_stack(bucket, entries, nr_entries, hash);
> +       /* Try to find again, to avoid concurrently inserting duplicates. */
> +       found = find_stack(bucket, entries, nr_entries, hash, depot_flags);
>         if (!found) {
>                 struct stack_record *new =
>                         depot_alloc_stack(entries, nr_entries, hash, &prealloc);
>
>                 if (new) {
> -                       list_add(&new->list, bucket);
> +                       /*
> +                        * This releases the stack record into the bucket and
> +                        * makes it visible to readers in find_stack().
> +                        */
> +                       list_add_rcu(&new->hash_list, bucket);
>                         found = new;
>                 }
> -       } else {
> -               if (depot_flags & STACK_DEPOT_FLAG_GET)
> -                       refcount_inc(&found->count);
> +       } else if (prealloc) {
>                 /*
>                  * Stack depot already contains this stack trace, but let's
>                  * keep the preallocated memory for future.
>                  */
> -               if (prealloc)
> -                       depot_keep_new_pool(&prealloc);
> +               depot_keep_new_pool(&prealloc);
>         }
>
>         printk_deferred_exit();
> -       write_unlock_irqrestore(&pool_rwlock, flags);
> +       raw_spin_unlock_irqrestore(&pool_lock, flags);
>  exit:
>         if (prealloc) {
>                 /* Stack depot didn't use this memory, free it. */
> @@ -592,7 +647,6 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>                                unsigned long **entries)
>  {
>         struct stack_record *stack;
> -       unsigned long flags;
>
>         *entries = NULL;
>         /*
> @@ -604,13 +658,12 @@ unsigned int stack_depot_fetch(depot_stack_handle_t handle,
>         if (!handle || stack_depot_disabled)
>                 return 0;
>
> -       read_lock_irqsave(&pool_rwlock, flags);
> -       printk_deferred_enter();
> -
>         stack = depot_fetch_stack(handle);
> -
> -       printk_deferred_exit();
> -       read_unlock_irqrestore(&pool_rwlock, flags);
> +       /*
> +        * Should never be NULL, otherwise this is a use-after-put.
> +        */
> +       if (WARN_ON(!stack))
> +               return 0;
>
>         *entries = stack->entries;
>         return stack->size;
> @@ -620,29 +673,20 @@ EXPORT_SYMBOL_GPL(stack_depot_fetch);
>  void stack_depot_put(depot_stack_handle_t handle)
>  {
>         struct stack_record *stack;
> -       unsigned long flags;
>
>         if (!handle || stack_depot_disabled)
>                 return;
>
> -       write_lock_irqsave(&pool_rwlock, flags);
> -       printk_deferred_enter();
> -
>         stack = depot_fetch_stack(handle);
> +       /*
> +        * Should always be able to find the stack record, otherwise this is an
> +        * unbalanced put attempt.
> +        */
>         if (WARN_ON(!stack))
> -               goto out;
> -
> -       if (refcount_dec_and_test(&stack->count)) {
> -               /* Unlink stack from the hash table. */
> -               list_del(&stack->list);
> +               return;
>
> -               /* Free stack. */
> +       if (refcount_dec_and_test(&stack->count))
>                 depot_free_stack(stack);
> -       }
> -
> -out:
> -       printk_deferred_exit();
> -       write_unlock_irqrestore(&pool_rwlock, flags);
>  }
>  EXPORT_SYMBOL_GPL(stack_depot_put);
>
> --
> 2.43.0.275.g3460e3d667-goog
>

Looks good to me from the functional perspective (modulo the
clarification comments I left above), but it would be great to get a
review from someone with a better understanding of the low-level
synchronization primitives.

Thank you!
  

On Fri, Jan 12, 2024 at 11:15:05PM +0100, Marco Elver wrote:
> +		/*
> +		 * Stack traces of size 0 are never saved, and we can simply use
> +		 * the size field as an indicator if this is a new unused stack
> +		 * record in the freelist.
> +		 */
> +		stack->size = 0;

I would use WRITE_ONCE here too, at least for TSan.

> +		return NULL;
> +
> +	/*
> +	 * We maintain the invariant that the elements in front are least
> +	 * recently used, and are therefore more likely to be associated with an
> +	 * RCU grace period in the past. Consequently it is sufficient to only
> +	 * check the first entry.
> +	 */
> +	stack = list_first_entry(&free_stacks, struct stack_record, free_list);
> +	if (stack->size && !poll_state_synchronize_rcu(stack->rcu_state))

READ_ONCE (also for TSan, and might be safer long term in case the
compiler considers some fancy code transformation)

> +		return NULL;
>  
> +		stack = depot_pop_free();
> +		if (WARN_ON(!stack))

Won't you get nesting problems here if this triggers due to the print?
I assume the nmi safe printk won't consider it like an NMI.

>  	counters[DEPOT_COUNTER_FREELIST_SIZE]++;
>  	counters[DEPOT_COUNTER_FREES]++;
>  	counters[DEPOT_COUNTER_INUSE]--;
> +
> +	printk_deferred_exit();

Ah this handles the WARN_ON? Should be ok then.

-Andi
  

On Sat, 13 Jan 2024 at 02:24, Andi Kleen <ak@linux.intel.com> wrote:
>
> On Fri, Jan 12, 2024 at 11:15:05PM +0100, Marco Elver wrote:
> > +             /*
> > +              * Stack traces of size 0 are never saved, and we can simply use
> > +              * the size field as an indicator if this is a new unused stack
> > +              * record in the freelist.
> > +              */
> > +             stack->size = 0;
>
> I would use WRITE_ONCE here too, at least for TSan.

This is written with the pool_lock held.

> > +             return NULL;
> > +
> > +     /*
> > +      * We maintain the invariant that the elements in front are least
> > +      * recently used, and are therefore more likely to be associated with an
> > +      * RCU grace period in the past. Consequently it is sufficient to only
> > +      * check the first entry.
> > +      */
> > +     stack = list_first_entry(&free_stacks, struct stack_record, free_list);
> > +     if (stack->size && !poll_state_synchronize_rcu(stack->rcu_state))
>
> READ_ONCE (also for TSan, and might be safer long term in case the
> compiler considers some fancy code transformation)

And this is also only read with the pool_lock held, so it's impossible
that there'd be a data race due to size. (And if there is a data race,
I'd want KCSAN to tell us because that'd be a bug then.)
depot_pop_free() can't be used w/o the lock because it's manipulating
the freelist.
To be sure, I'm adding a lockdep_assert_held() to depot_pop_free().

> > +             return NULL;
> >
> > +             stack = depot_pop_free();
> > +             if (WARN_ON(!stack))
>
> Won't you get nesting problems here if this triggers due to the print?
> I assume the nmi safe printk won't consider it like an NMI.
>
> >       counters[DEPOT_COUNTER_FREELIST_SIZE]++;
> >       counters[DEPOT_COUNTER_FREES]++;
> >       counters[DEPOT_COUNTER_INUSE]--;
> > +
> > +     printk_deferred_exit();
>
> Ah this handles the WARN_ON? Should be ok then.

Yes, the pool_lock critical sections are surrounded by printk_deferred.

Thanks,
-- Marco
  

> This function is only refilling the freelist. Readers don't see it yet
> because it's in none of the hash table buckets. The freelist is only
> ever accessed under the lock.
> 
> Once an entry is allocated from the freelist, its size is overwritten
> with something non-zero (since it then contains a stack trace). Those
> updates are released into the right hash table bucket with
> list_add_rcu() (which implies a release).
> 
> Am I missing something else?

It's probably ok semantically here, but at least I would be consistent with
using the macro for a specific field.

-Andi