@@ -6,6 +6,7 @@
#include <linux/errno.h>
#include <linux/bug.h>
#include "printk_ringbuffer.h"
+#include "internal.h"
/**
* DOC: printk_ringbuffer overview
@@ -303,6 +304,9 @@
*
* desc_push_tail:B / desc_reserve:D
* set descriptor reusable (state), then push descriptor tail (id)
+ *
+ * desc_update_last_finalized:A / desc_last_finalized_seq:A
+ * store finalized record, then set new highest finalized sequence number
*/
#define DATA_SIZE(data_ring) _DATA_SIZE((data_ring)->size_bits)
@@ -1441,20 +1445,144 @@ bool prb_reserve_in_last(struct prb_reserved_entry *e, struct printk_ringbuffer
return false;
}
+#ifdef CONFIG_64BIT
+
+#define __u64seq_to_ulseq(u64seq) (u64seq)
+#define __ulseq_to_u64seq(ulseq) (ulseq)
+
+#else /* CONFIG_64BIT */
+
+static u64 prb_first_seq(struct printk_ringbuffer *rb);
+
+#define __u64seq_to_ulseq(u64seq) ((u32)u64seq)
+static inline u64 __ulseq_to_u64seq(u32 ulseq)
+{
+ u64 rb_first_seq = prb_first_seq(prb);
+ u64 seq;
+
+ /*
+ * The provided sequence is only the lower 32 bits of the ringbuffer
+ * sequence. It needs to be expanded to 64bit. Get the first sequence
+ * number from the ringbuffer and fold it.
+ */
+ seq = rb_first_seq - ((u32)rb_first_seq - ulseq);
+
+ return seq;
+}
+
+#endif /* CONFIG_64BIT */
+
+/*
+ * @last_finalized_seq value guarantees that all records up to and including
+ * this sequence number are finalized and can be read. The only exception are
+ * too old records which have already been overwritten.
+ *
+ * It is also guaranteed that @last_finalized_seq only increases.
+ *
+ * Be aware that finalized records following non-finalized records are not
+ * reported because they are not yet available to the reader. For example,
+ * a new record stored via printk() will not be available to a printer if
+ * it follows a record that has not been finalized yet. However, once that
+ * non-finalized record becomes finalized, @last_finalized_seq will be
+ * appropriately updated and the full set of finalized records will be
+ * available to the printer. And since each printk() caller will either
+ * directly print or trigger deferred printing of all available unprinted
+ * records, all printk() messages will get printed.
+ */
+static u64 desc_last_finalized_seq(struct prb_desc_ring *desc_ring)
+{
+ unsigned long ulseq;
+
+ /*
+ * Guarantee the sequence number is loaded before loading the
+ * associated record in order to guarantee that the record can be
+ * seen by this CPU. This pairs with desc_update_last_finalized:A.
+ */
+ ulseq = atomic_long_read_acquire(&desc_ring->last_finalized_seq
+ ); /* LMM(desc_last_finalized_seq:A) */
+
+ return __ulseq_to_u64seq(ulseq);
+}
+
+static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
+ struct printk_record *r, unsigned int *line_count);
+
+/*
+ * Check if there are records directly following @last_finalized_seq that are
+ * finalized. If so, update @last_finalized_seq to the latest of these
+ * records. It is not allowed to skip over records that are not yet finalized.
+ */
+static void desc_update_last_finalized(struct printk_ringbuffer *rb)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ u64 old_seq = desc_last_finalized_seq(desc_ring);
+ unsigned long oldval;
+ unsigned long newval;
+ u64 finalized_seq;
+ u64 try_seq;
+
+try_again:
+ finalized_seq = old_seq;
+ try_seq = finalized_seq + 1;
+
+ /* Try to find later finalized records. */
+ while (_prb_read_valid(rb, &try_seq, NULL, NULL)) {
+ finalized_seq = try_seq;
+ try_seq++;
+ }
+
+ /* No update needed if no later finalized record was found. */
+ if (finalized_seq == old_seq)
+ return;
+
+ oldval = __u64seq_to_ulseq(old_seq);
+ newval = __u64seq_to_ulseq(finalized_seq);
+
+ /*
+ * Set the sequence number of a later finalized record that has been
+ * seen.
+ *
+ * Guarantee the record data is visible to other CPUs before storing
+ * its sequence number. This pairs with desc_last_finalized_seq:A.
+ *
+ * Memory barrier involvement:
+ *
+ * If desc_last_finalized_seq:A reads from
+ * desc_update_last_finalized:A, then desc_read:A reads from
+ * _prb_commit:B.
+ *
+ * Relies on:
+ *
+ * RELEASE from _prb_commit:B to desc_update_last_finalized:A
+ * matching
+ * ACQUIRE from desc_last_finalized_seq:A to desc_read:A
+ *
+ * Note: _prb_commit:B and desc_update_last_finalized:A can be
+ * different CPUs. However, the desc_update_last_finalized:A
+ * CPU (which performs the release) must have previously seen
+ * _prb_commit:B.
+ */
+ if (!atomic_long_try_cmpxchg_release(&desc_ring->last_finalized_seq,
+ &oldval, newval)) { /* LMM(desc_update_last_finalized:A) */
+ old_seq = __ulseq_to_u64seq(oldval);
+ goto try_again;
+ }
+}
+
/*
* Attempt to finalize a specified descriptor. If this fails, the descriptor
* is either already final or it will finalize itself when the writer commits.
*/
-static void desc_make_final(struct prb_desc_ring *desc_ring, unsigned long id)
+static void desc_make_final(struct printk_ringbuffer *rb, unsigned long id)
{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
unsigned long prev_state_val = DESC_SV(id, desc_committed);
struct prb_desc *d = to_desc(desc_ring, id);
- atomic_long_cmpxchg_relaxed(&d->state_var, prev_state_val,
- DESC_SV(id, desc_finalized)); /* LMM(desc_make_final:A) */
-
- /* Best effort to remember the last finalized @id. */
- atomic_long_set(&desc_ring->last_finalized_id, id);
+ if (atomic_long_try_cmpxchg_relaxed(&d->state_var, &prev_state_val,
+ DESC_SV(id, desc_finalized))) { /* LMM(desc_make_final:A) */
+ desc_update_last_finalized(rb);
+ }
}
/**
@@ -1550,7 +1678,7 @@ bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
* readers. (For seq==0 there is no previous descriptor.)
*/
if (info->seq > 0)
- desc_make_final(desc_ring, DESC_ID(id - 1));
+ desc_make_final(rb, DESC_ID(id - 1));
r->text_buf = data_alloc(rb, r->text_buf_size, &d->text_blk_lpos, id);
/* If text data allocation fails, a data-less record is committed. */
@@ -1643,7 +1771,7 @@ void prb_commit(struct prb_reserved_entry *e)
*/
head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_commit:A) */
if (head_id != e->id)
- desc_make_final(desc_ring, e->id);
+ desc_make_final(e->rb, e->id);
}
/**
@@ -1663,12 +1791,9 @@ void prb_commit(struct prb_reserved_entry *e)
*/
void prb_final_commit(struct prb_reserved_entry *e)
{
- struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
-
_prb_commit(e, desc_finalized);
- /* Best effort to remember the last finalized @id. */
- atomic_long_set(&desc_ring->last_finalized_id, e->id);
+ desc_update_last_finalized(e->rb);
}
/*
@@ -2008,7 +2133,9 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
* newest sequence number available to readers will be.
*
* This provides readers a sequence number to jump to if all currently
- * available records should be skipped.
+ * available records should be skipped. It is guaranteed that all records
+ * previous to the returned value have been finalized and are (or were)
+ * available to the reader.
*
* Context: Any context.
* Return: The sequence number of the next newest (not yet available) record
@@ -2017,33 +2144,19 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
u64 prb_next_seq(struct printk_ringbuffer *rb)
{
struct prb_desc_ring *desc_ring = &rb->desc_ring;
- enum desc_state d_state;
- unsigned long id;
u64 seq;
- /* Check if the cached @id still points to a valid @seq. */
- id = atomic_long_read(&desc_ring->last_finalized_id);
- d_state = desc_read(desc_ring, id, NULL, &seq, NULL);
+ seq = desc_last_finalized_seq(desc_ring);
- if (d_state == desc_finalized || d_state == desc_reusable) {
- /*
- * Begin searching after the last finalized record.
- *
- * On 0, the search must begin at 0 because of hack#2
- * of the bootstrapping phase it is not known if a
- * record at index 0 exists.
- */
- if (seq != 0)
- seq++;
- } else {
- /*
- * The information about the last finalized sequence number
- * has gone. It should happen only when there is a flood of
- * new messages and the ringbuffer is rapidly recycled.
- * Give up and start from the beginning.
- */
- seq = 0;
- }
+ /*
+ * Begin searching after the last finalized record.
+ *
+ * On 0, the search must begin at 0 because of hack#2
+ * of the bootstrapping phase it is not known if a
+ * record at index 0 exists.
+ */
+ if (seq != 0)
+ seq++;
/*
* The information about the last finalized @seq might be inaccurate.
@@ -2085,7 +2198,7 @@ void prb_init(struct printk_ringbuffer *rb,
rb->desc_ring.infos = infos;
atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
- atomic_long_set(&rb->desc_ring.last_finalized_id, DESC0_ID(descbits));
+ atomic_long_set(&rb->desc_ring.last_finalized_seq, 0);
rb->text_data_ring.size_bits = textbits;
rb->text_data_ring.data = text_buf;
@@ -75,7 +75,7 @@ struct prb_desc_ring {
struct printk_info *infos;
atomic_long_t head_id;
atomic_long_t tail_id;
- atomic_long_t last_finalized_id;
+ atomic_long_t last_finalized_seq;
};
/*
@@ -259,7 +259,7 @@ static struct printk_ringbuffer name = { \
.infos = &_##name##_infos[0], \
.head_id = ATOMIC_INIT(DESC0_ID(descbits)), \
.tail_id = ATOMIC_INIT(DESC0_ID(descbits)), \
- .last_finalized_id = ATOMIC_INIT(DESC0_ID(descbits)), \
+ .last_finalized_seq = ATOMIC_INIT(0), \
}, \
.text_data_ring = { \
.size_bits = (avgtextbits) + (descbits), \