[v7,1/5] lib: objpool added: ring-array based lockless MPMC queue

  The object pool is a scalable implementaion of high performance queue
for objects allocation and reclamation, such as kretprobe instances.

With leveraging per-cpu ring-array to mitigate the hot spots of memory
contention, it could deliver near-linear scalability for high parallel
scenarios. The ring-array is compactly managed in a single cache-line
to benefit from warmed L1 cache for most cases (<= 4 objects per-core).
The body of pre-allocated objects is stored in continuous cache-lines
just after the ring-array.

The object pool is interrupt safe. Both allocation and reclamation
(object pop and push operations) can be preemptible or interruptable.

It's best suited for following cases:
1) Memory allocation or reclamation are prohibited or too expensive
2) Consumers are of different priorities, such as irqs and threads

Limitations:
1) Maximum objects (capacity) is determined during pool initializing
2) The memory of objects won't be freed until the poll is finalized
3) Object allocation (pop) may fail after trying all cpu slots
4) Object reclamation (push) won't fail but may take long time to
   finish for imbalanced scenarios. You can try larger max_entries
   to mitigate, or ( >= CPUS * nr_objs) to avoid

Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
---
 include/linux/objpool.h | 109 ++++++++++++++
 lib/Makefile            |   2 +-
 lib/objpool.c           | 320 ++++++++++++++++++++++++++++++++++++++++
 3 files changed, 430 insertions(+), 1 deletion(-)
 create mode 100644 include/linux/objpool.h
 create mode 100644 lib/objpool.c
  

Hi Matt,

Thanks for your update! I reviewed it and have some comments and questions.

BTW, from the next version, can you Cc the series to linux-trace-kernel@vger
mailing list too?

http://vger.kernel.org/vger-lists.html#linux-trace-kernel

On Mon, 12 Dec 2022 20:31:49 +0800
wuqiang <wuqiang.matt@bytedance.com> wrote:

> The object pool is a scalable implementaion of high performance queue
> for objects allocation and reclamation, such as kretprobe instances.
> 
> With leveraging per-cpu ring-array to mitigate the hot spots of memory
> contention, it could deliver near-linear scalability for high parallel
> scenarios. The ring-array is compactly managed in a single cache-line
> to benefit from warmed L1 cache for most cases (<= 4 objects per-core).
> The body of pre-allocated objects is stored in continuous cache-lines
> just after the ring-array.
> 
> The object pool is interrupt safe. Both allocation and reclamation
> (object pop and push operations) can be preemptible or interruptable.
> 
> It's best suited for following cases:
> 1) Memory allocation or reclamation are prohibited or too expensive
> 2) Consumers are of different priorities, such as irqs and threads
> 
> Limitations:
> 1) Maximum objects (capacity) is determined during pool initializing
> 2) The memory of objects won't be freed until the poll is finalized
> 3) Object allocation (pop) may fail after trying all cpu slots
> 4) Object reclamation (push) won't fail but may take long time to
>    finish for imbalanced scenarios. You can try larger max_entries
>    to mitigate, or ( >= CPUS * nr_objs) to avoid
> 
> Signed-off-by: wuqiang <wuqiang.matt@bytedance.com>
> ---
>  include/linux/objpool.h | 109 ++++++++++++++
>  lib/Makefile            |   2 +-
>  lib/objpool.c           | 320 ++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 430 insertions(+), 1 deletion(-)
>  create mode 100644 include/linux/objpool.h
>  create mode 100644 lib/objpool.c
> 
> diff --git a/include/linux/objpool.h b/include/linux/objpool.h
> new file mode 100644
> index 000000000000..922e1bc96f2b
> --- /dev/null
> +++ b/include/linux/objpool.h
> @@ -0,0 +1,109 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef _LINUX_OBJPOOL_H
> +#define _LINUX_OBJPOOL_H
> +
> +#include <linux/types.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC queue
> + *
> + * Copyright: wuqiang.matt@bytedance.com
> + *
> + * The object pool is a scalable implementaion of high performance queue
> + * for objects allocation and reclamation, such as kretprobe instances.
> + *
> + * With leveraging per-cpu ring-array to mitigate the hot spots of memory
> + * contention, it could deliver near-linear scalability for high parallel
> + * scenarios. The ring-array is compactly managed in a single cache-line
> + * to benefit from warmed L1 cache for most cases (<= 4 objects per-core).
> + * The body of pre-allocated objects is stored in continuous cache-lines
> + * just after the ring-array.
> + *
> + * The object pool is interrupt safe. Both allocation and reclamation
> + * (object pop and push operations) can be preemptible or interruptable.
> + *
> + * It's best suited for following cases:
> + * 1) Memory allocation or reclamation are prohibited or too expensive
> + * 2) Consumers are of different priorities, such as irqs and threads
> + *
> + * Limitations:
> + * 1) Maximum objects (capacity) is determined during pool initializing
> + * 2) The memory of objects won't be freed until the poll is finalized
> + * 3) Object allocation (pop) may fail after trying all cpu slots
> + */
> +
> +/*
> + * objpool_slot: per-cpu ring array
> + *
> + * Represents a cpu-local array-based ring buffer, its size is specialized
> + * during initialization of object pool.
> + *
> + * The objpool_slot is allocated from local memory for NUMA system, and to
> + * be kept compact in a single cacheline. ages[] is stored just after the
> + * body of objpool_slot, and then entries[]. The Array of ages[] describes
> + * revision of each item, solely used to avoid ABA. And array of entries[]
> + * contains the pointers of objects.
> + *
> + * The default size of objpool_slot is a single cache-line, aka. 64 bytes.
> + *
> + * 64bit:
> + *        4      8      12     16        32                 64
> + * | head | tail | size | mask | ages[4] | ents[4]: (8 * 4) | objects
> + *
> + * 32bit:
> + *        4      8      12     16        32        48       64
> + * | head | tail | size | mask | ages[4] | ents[4] | unused | objects
> + *
> + */
> +
> +struct objpool_slot {
> +	uint32_t                head;	/* head of ring array */
> +	uint32_t                tail;	/* tail of ring array */
> +	uint32_t                size;	/* array size, pow of 2 */
> +	uint32_t                mask;	/* size - 1 */
> +} __packed;
> +
> +struct objpool_head;
> +
> +/* caller-specified callback for object initial setup, only called once */

This comment is a bit confusing. it is "called once for each object", right?

> +typedef int (*objpool_init_obj_cb)(void *obj, void *context);
> +
> +/* caller-specified cleanup callback for objpool destruction */
> +typedef int (*objpool_fini_cb)(struct objpool_head *head, void *context);
> +
> +/*
> + * objpool_head: object pooling metadata
> + */
> +
> +struct objpool_head {
> +	int                     obj_size;	/* object & element size */
> +	int                     nr_objs;	/* total objs (to be pre-allocated) */
> +	int                     nr_cpus;	/* nr_cpu_ids */
> +	int                     capacity;	/* max objects per cpuslot */
> +	gfp_t                   gfp;		/* gfp flags for kmalloc & vmalloc */
> +	unsigned long           flags;		/* flags for objpool management */
> +	struct objpool_slot   **cpu_slots;	/* array of percpu slots */
> +	int                    *slot_sizes;	/* size in bytes of slots */
> +	objpool_fini_cb         release;	/* resource cleanup callback */
> +	void                   *context;	/* caller-provided context */
> +};
> +
> +#define OBJPOOL_FROM_VMALLOC	(0x800000000)	/* objpool allocated from vmalloc area */
> +#define OBJPOOL_HAVE_OBJECTS	(0x400000000)	/* objects allocated along with objpool */
> +
> +/* initialize object pool and pre-allocate objects */
> +int objpool_init(struct objpool_head *head, int nr_objs, int object_size,
> +		 gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		 objpool_fini_cb release);
> +
> +/* allocate an object from objects pool */
> +void *objpool_pop(struct objpool_head *head);
> +
> +/* reclaim an object to objects pool */
> +int objpool_push(void *node, struct objpool_head *head);
> +
> +/* cleanup the whole object pool (objects including) */
> +void objpool_fini(struct objpool_head *head);
> +
> +#endif /* _LINUX_OBJPOOL_H */
> diff --git a/lib/Makefile b/lib/Makefile
> index 59bd7c2f793a..f23d9c4fe639 100644
> --- a/lib/Makefile
> +++ b/lib/Makefile
> @@ -34,7 +34,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
>  	 is_single_threaded.o plist.o decompress.o kobject_uevent.o \
>  	 earlycpio.o seq_buf.o siphash.o dec_and_lock.o \
>  	 nmi_backtrace.o win_minmax.o memcat_p.o \
> -	 buildid.o
> +	 buildid.o objpool.o
>  
>  lib-$(CONFIG_PRINTK) += dump_stack.o
>  lib-$(CONFIG_SMP) += cpumask.o
> diff --git a/lib/objpool.c b/lib/objpool.c
> new file mode 100644
> index 000000000000..bab8b27e75d7
> --- /dev/null
> +++ b/lib/objpool.c
> @@ -0,0 +1,320 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/objpool.h>
> +#include <linux/slab.h>
> +#include <linux/vmalloc.h>
> +#include <linux/atomic.h>
> +#include <linux/prefetch.h>
> +#include <linux/cpumask.h>
> +
> +/*
> + * objpool: ring-array based lockless MPMC/FIFO queues
> + *
> + * Copyright: wuqiang.matt@bytedance.com
> + */
> +
> +/* compute the suitable num of objects to be managed by slot */
> +static inline int objpool_nobjs(int size)
> +{
> +	return rounddown_pow_of_two((size - sizeof(struct objpool_slot)) /
> +			(sizeof(uint32_t) + sizeof(void *)));
> +}
> +
> +#define SLOT_AGES(s) ((uint32_t *)((char *)(s) + sizeof(struct objpool_slot)))
> +#define SLOT_ENTS(s) ((void **)((char *)(s) + sizeof(struct objpool_slot) + \
> +			sizeof(uint32_t) * (s)->size))
> +#define SLOT_OBJS(s) ((void *)((char *)(s) + sizeof(struct objpool_slot) + \
> +			(sizeof(uint32_t) + sizeof(void *)) * (s)->size))
> +#define SLOT_CORE(n) cpumask_nth((n) % num_possible_cpus(), cpu_possible_mask)
> +
> +/* allocate and initialize percpu slots */
> +static inline int

nit: I don't think this needs to be inlined, since it is only
used once and not on a hot path. I would suggest making it just
a 'static' function and letting the compiler decide whether
to inline it or not.

> +objpool_init_percpu_slots(struct objpool_head *head, int nobjs,
> +			void *context, objpool_init_obj_cb objinit)
> +{
> +	int i, j, n, size, objsz, cpu = 0, nents = head->capacity;

'nents' and 'n' is a bit confusing. please use 'capacity' or always
use 'head->capacity'.

> +
> +	/* aligned object size by sizeof(void *) */
> +	objsz = ALIGN(head->obj_size, sizeof(void *));
> +	/* shall we allocate objects along with objpool_slot */
> +	if (objsz)
> +		head->flags |= OBJPOOL_HAVE_OBJECTS;
> +
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		struct objpool_slot *os;
> +
> +		/* skip the cpus which could never be present */
> +		if (!cpu_possible(i))
> +			continue;

You can use ;

	for_each_possible_cpu(i) {

> +
> +		/* compute how many objects to be managed by this slot */
> +		n = nobjs / num_possible_cpus();
> +		if (cpu < (nobjs % num_possible_cpus()))
> +			n++;
> +		size = sizeof(struct objpool_slot) + sizeof(void *) * nents +
> +		       sizeof(uint32_t) * nents + objsz * n;
> +
> +		/* decide memory area for cpu-slot allocation */
> +		if (!cpu && !(head->gfp & GFP_ATOMIC) && size > PAGE_SIZE / 2)
> +			head->flags |= OBJPOOL_FROM_VMALLOC;

Why 'PAGE_SIZE / 2' ?

> +
> +		/* allocate percpu slot & objects from local memory */
> +		if (head->flags & OBJPOOL_FROM_VMALLOC)
> +			os = __vmalloc_node(size, sizeof(void *), head->gfp,
> +				cpu_to_node(i), __builtin_return_address(0));
> +		else
> +			os = kmalloc_node(size, head->gfp, cpu_to_node(i));
> +		if (!os)
> +			return -ENOMEM;
> +
> +		/* initialize percpu slot for the i-th slot */
> +		memset(os, 0, size);
> +		os->size = head->capacity;
> +		os->mask = os->size - 1;
> +		head->cpu_slots[i] = os;
> +		head->slot_sizes[i] = size;
> +		cpu = cpu + 1;
> +
> +		/*
> +		 * start from 2nd round to avoid conflict of 1st item.
> +		 * we assume that the head item is ready for retrieval
> +		 * iff head is equal to ages[head & mask]. but ages is
> +		 * initialized as 0, so in view of the caller of pop(),
> +		 * the 1st item (0th) is always ready, but fact could
> +		 * be: push() is stalled before the final update, thus
> +		 * the item being inserted will be lost forever.
> +		 */
> +		os->head = os->tail = head->capacity;
> +
> +		if (!objsz)
> +			continue;
> +
> +		for (j = 0; j < n; j++) {
> +			uint32_t *ages = SLOT_AGES(os);
> +			void **ents = SLOT_ENTS(os);
> +			void *obj = SLOT_OBJS(os) + j * objsz;
> +			uint32_t ie = os->tail & os->mask;
> +
> +			/* perform object initialization */
> +			if (objinit) {
> +				int rc = objinit(obj, context);
> +				if (rc)
> +					return rc;
> +			}
> +
> +			/* add obj into the ring array */
> +			ents[ie] = obj;
> +			ages[ie] = os->tail;
> +			os->tail++;
> +			head->nr_objs++;
> +		}
> +	}
> +
> +	return 0;
> +}
> +
> +/* cleanup all percpu slots of the object pool */
> +static inline void objpool_fini_percpu_slots(struct objpool_head *head)

This is also no need to be inlined.

> +{
> +	int i;
> +
> +	if (!head->cpu_slots)
> +		return;
> +
> +	for (i = 0; i < head->nr_cpus; i++) {
> +		if (!head->cpu_slots[i])
> +			continue;
> +		if (head->flags & OBJPOOL_FROM_VMALLOC)
> +			vfree(head->cpu_slots[i]);
> +		else
> +			kfree(head->cpu_slots[i]);
> +	}
> +	kfree(head->cpu_slots);
> +	head->cpu_slots = NULL;
> +	head->slot_sizes = NULL;
> +}
> +
> +/**
> + * objpool_init: initialize object pool and pre-allocate objects
> + *
> + * args:

At first, please write this in kernel-doc style.
See Documentation/doc-guide/kernel-doc.rst

> + * @head:    the object pool to be initialized, declared by caller
> + * @nr_objs: total objects to be pre-allocated by this object pool
> + * @object_size: size of an object, no objects pre-allocated if 0

This is a bit strange. If no objects pre-allocated, @nr_objs should
be 0 instead of (or both of) @object_size.

And anyway, if there is no actual use-case for non pre-allocated
feature (except for the test case, of course), I would suggest
dropping it from this first version.

> + * @gfp:     flags for memory allocation (via kmalloc or vmalloc)
> + * @context: user context for object initialization callback
> + * @objinit: object initialization callback for extra setting-up
> + * @release: cleanup callback for private objects/pool/context
> + *
> + * return:
> + *         0 for success, otherwise error code
> + *
> + * All pre-allocated objects are to be zeroed. Caller could do extra
> + * initialization in objinit callback. The objinit callback will be
> + * called once and only once after the slot allocation. Then objpool
> + * won't touch any content of the objects since then. It's caller's
> + * duty to perform reinitialization after object allocation (pop) or
> + * clearance before object reclamation (push) if required.
> + */
> +int objpool_init(struct objpool_head *head, int nr_objs, int object_size,
> +		gfp_t gfp, void *context, objpool_init_obj_cb objinit,
> +		objpool_fini_cb release)
> +{
> +	int nents, rc;
> +
> +	/* check input parameters */
> +	if (nr_objs <= 0 || object_size < 0)
> +		return -EINVAL;
> +
> +	/* calculate percpu slot size (rounded to pow of 2) */
> +	nents = max_t(int, roundup_pow_of_two(nr_objs),
> +			objpool_nobjs(L1_CACHE_BYTES));
> +
> +	/* initialize objpool head */
> +	memset(head, 0, sizeof(struct objpool_head));
> +	head->nr_cpus = nr_cpu_ids;
> +	head->obj_size = object_size;
> +	head->capacity = nents;
> +	head->gfp = gfp & ~__GFP_ZERO;
> +	head->context = context;
> +	head->release = release;
> +
> +	/* allocate array for percpu slots */
> +	head->cpu_slots = kzalloc(head->nr_cpus * sizeof(void *) +
> +			       head->nr_cpus * sizeof(uint32_t), head->gfp);

This looks wired. Please allocate the array for cpu_slots and slot_sizes
separately.

> +	if (!head->cpu_slots)
> +		return -ENOMEM;
> +	head->slot_sizes = (uint32_t *)&head->cpu_slots[head->nr_cpus];

And do not do this. If it allocates 2 arrays, we can use many debug
features to detect overrun, but this wired allocation will prevent it.

> +
> +	/* initialize per-cpu slots */
> +	rc = objpool_init_percpu_slots(head, nr_objs, context, objinit);
> +	if (rc)
> +		objpool_fini_percpu_slots(head);
> +
> +	return rc;
> +}
> +EXPORT_SYMBOL_GPL(objpool_init);
> +
> +/* adding object to slot tail, the given slot must NOT be full */
> +static inline int objpool_add_slot(void *obj, struct objpool_slot *os)
> +{
> +	uint32_t *ages = SLOT_AGES(os);
> +	void **ents = SLOT_ENTS(os);
> +	uint32_t tail = atomic_inc_return((atomic_t *)&os->tail) - 1;

No, don't cast u32 to atomic_t. 

> +
> +	WRITE_ONCE(ents[tail & os->mask], obj);
> +
> +	/* order matters: obj must be updated before tail updating */
> +	smp_store_release(&ages[tail & os->mask], tail);
> +	return 0;
> +}
> +
> +/**
> + * objpool_push: reclaim the object and return back to objects pool

Ditto, please use kernel-doc.

> + *
> + * args:
> + * @obj:  object pointer to be pushed to object pool
> + * @head: object pool
> + *
> + * return:
> + *     0 or error code: it fails only when objects pool are full
> + *
> + * objpool_push is non-blockable, and can be nested
> + */
> +int objpool_push(void *obj, struct objpool_head *head)
> +{
> +	int cpu = raw_smp_processor_id();
> +
> +	return objpool_add_slot(obj, head->cpu_slots[cpu]);
> +}
> +EXPORT_SYMBOL_GPL(objpool_push);
> +
> +/* try to retrieve object from slot */
> +static inline void *objpool_try_get_slot(struct objpool_slot *os)
> +{
> +	uint32_t *ages = SLOT_AGES(os);
> +	void **ents = SLOT_ENTS(os);
> +	/* do memory load of head to local head */
> +	uint32_t head = smp_load_acquire(&os->head);
> +
> +	/* loop if slot isn't empty */
> +	while (head != READ_ONCE(os->tail)) {
> +		uint32_t id = head & os->mask, prev = head;
> +
> +		/* do prefetching of object ents */
> +		prefetch(&ents[id]);
> +
> +		/*
> +		 * check whether this item was ready for retrieval ? There's
> +		 * possibility * in theory * we might retrieve wrong object,
> +		 * in case ages[id] overflows when current task is sleeping,
> +		 * but it will take very very long to overflow an uint32_t
> +		 */

We should make sure it is safe in theory. The hot path can be loose but
it must be ensured before use. OS can be used very long time in some time
(e.g. 10 years) and uint32 is too short ... (uint64 is OK)

> +		if (smp_load_acquire(&ages[id]) == head) {
> +			/* node must have been udpated by push() */
> +			void *node = READ_ONCE(ents[id]);
> +			/* commit and move forward head of the slot */
> +			if (try_cmpxchg_release(&os->head, &head, head + 1))
> +				return node;
> +		}
> +
> +		/* re-load head from memory and continue trying */
> +		head = READ_ONCE(os->head);
> +		/*
> +		 * head stays unchanged, so it's very likely current pop()
> +		 * just preempted/interrupted an ongoing push() operation
> +		 */
> +		if (head == prev)
> +			break;
> +	}
> +
> +	return NULL;
> +}
> +
> +/**
> + * objpool_pop: allocate an object from objects pool

Ditto.

Thank you,

> + *
> + * args:
> + * @head: object pool
> + *
> + * return:
> + *   object: NULL if failed (object pool is empty)
> + *
> + * objpool_pop can be nested, so can be used in any context.
> + */
> +void *objpool_pop(struct objpool_head *head)
> +{
> +	int i, cpu = raw_smp_processor_id();
> +	void *obj = NULL;
> +
> +	for (i = 0; i < num_possible_cpus(); i++) {
> +		obj = objpool_try_get_slot(head->cpu_slots[cpu]);
> +		if (obj)
> +			break;
> +		cpu = cpumask_next_wrap(cpu, cpu_possible_mask, -1, 1);
> +	}
> +
> +	return obj;
> +}
> +EXPORT_SYMBOL_GPL(objpool_pop);
> +
> +/**
> + * objpool_fini: cleanup the whole object pool (releasing all objects)
> + *
> + * args:
> + * @head: object pool to be released
> + *
> + */
> +void objpool_fini(struct objpool_head *head)
> +{
> +	if (!head->cpu_slots)
> +		return;
> +
> +	/* release percpu slots */
> +	objpool_fini_percpu_slots(head);
> +
> +	/* call user's cleanup callback if provided */
> +	if (head->release)
> +		head->release(head, head->context);
> +}
> +EXPORT_SYMBOL_GPL(objpool_fini);
> -- 
> 2.34.1
>
  

Hi,

On Fri, 23 Dec 2022 00:47:01 +0900
Masami Hiramatsu (Google) <mhiramat@kernel.org> wrote:

> > +/* try to retrieve object from slot */
> > +static inline void *objpool_try_get_slot(struct objpool_slot *os)
> > +{
> > +	uint32_t *ages = SLOT_AGES(os);
> > +	void **ents = SLOT_ENTS(os);
> > +	/* do memory load of head to local head */
> > +	uint32_t head = smp_load_acquire(&os->head);
> > +
> > +	/* loop if slot isn't empty */
> > +	while (head != READ_ONCE(os->tail)) {
> > +		uint32_t id = head & os->mask, prev = head;
> > +
> > +		/* do prefetching of object ents */
> > +		prefetch(&ents[id]);
> > +
> > +		/*
> > +		 * check whether this item was ready for retrieval ? There's
> > +		 * possibility * in theory * we might retrieve wrong object,
> > +		 * in case ages[id] overflows when current task is sleeping,
> > +		 * but it will take very very long to overflow an uint32_t
> > +		 */
> 
> We should make sure it is safe in theory. The hot path can be loose but
> it must be ensured before use. OS can be used very long time in some time
> (e.g. 10 years) and uint32 is too short ... (uint64 is OK)

OK, I understand what you concerned here. This means that the ages[id]
overflows *and* back to the same value during here. But must objpool_pop()
be called under preempt disabled? If not, it should be (and that must not
be a big overhead).

> > +		if (smp_load_acquire(&ages[id]) == head) {
> > +			/* node must have been udpated by push() */

Please correct me. In my understanding, since the size of ents[] is always
bigger than nr_objs, (tail & mask) == (head & mask) only if the ents[] is
full and no free object (thus no push() happens) or ents[] is empty
(in this case ages[id] != head). Thus the node is not updated if below
cmpxchg is succeeded.

> > +			void *node = READ_ONCE(ents[id]);
> > +			/* commit and move forward head of the slot */
> > +			if (try_cmpxchg_release(&os->head, &head, head + 1))
> > +				return node;
> > +		}
> > +
> > +		/* re-load head from memory and continue trying */
> > +		head = READ_ONCE(os->head);
> > +		/*
> > +		 * head stays unchanged, so it's very likely current pop()
> > +		 * just preempted/interrupted an ongoing push() operation

Since this can touch the other CPUs' slot, there should be another ongoing
push() running on the same slot (so no need to limit the preempt/interrupt
cases.) Also, this happens only when the push() is running on *the empty slot*.
Thus we can consider this is empty, and return NULL.

Can you update the comment above and make it clear that this exits here
because it is empty until ongoing push() is done.

Overall, some comments must be clear, but the code itself seems OK to me.

Thank you,

> > +		 */
> > +		if (head == prev)
> > +			break;
> > +	}
> > +
> > +	return NULL;
> > +}
> > +
> > +/**
> > + * objpool_pop: allocate an object from objects pool
> 
> Ditto.
> 
> Thank you,
> 
> > + *
> > + * args:
> > + * @head: object pool
> > + *
> > + * return:
> > + *   object: NULL if failed (object pool is empty)
> > + *
> > + * objpool_pop can be nested, so can be used in any context.
> > + */
> > +void *objpool_pop(struct objpool_head *head)
> > +{
> > +	int i, cpu = raw_smp_processor_id();
> > +	void *obj = NULL;
> > +
> > +	for (i = 0; i < num_possible_cpus(); i++) {
> > +		obj = objpool_try_get_slot(head->cpu_slots[cpu]);
> > +		if (obj)
> > +			break;
> > +		cpu = cpumask_next_wrap(cpu, cpu_possible_mask, -1, 1);
> > +	}
> > +
> > +	return obj;
> > +}
> > +EXPORT_SYMBOL_GPL(objpool_pop);
> > +
> > +/**
> > + * objpool_fini: cleanup the whole object pool (releasing all objects)
> > + *
> > + * args:
> > + * @head: object pool to be released
> > + *
> > + */
> > +void objpool_fini(struct objpool_head *head)
> > +{
> > +	if (!head->cpu_slots)
> > +		return;
> > +
> > +	/* release percpu slots */
> > +	objpool_fini_percpu_slots(head);
> > +
> > +	/* call user's cleanup callback if provided */
> > +	if (head->release)
> > +		head->release(head, head->context);
> > +}
> > +EXPORT_SYMBOL_GPL(objpool_fini);
> > -- 
> > 2.34.1
> > 
> 
> 
> -- 
> Masami Hiramatsu (Google) <mhiramat@kernel.org>