[PATCH 6.7 218/713] printk: Wait for all reserved records with pr_flush()

Sasha Levin
2024-03-24T22:50:56Z

From: John Ogness <john.ogness@linutronix.de>

[ Upstream commit ac7d7844c64d15603daa3e905a311ddcfbb4bc91 ]

Currently pr_flush() will only wait for records that were
available to readers at the time of the call (using
prb_next_seq()). But there may be more records (non-finalized)
that have following finalized records. pr_flush() should wait
for these to print as well. Particularly because any trailing
finalized records may be the messages that the calling context
wants to ensure are printed.

Add a new ringbuffer function prb_next_reserve_seq() to return
the sequence number following the most recently reserved record.
This guarantees that pr_flush() will wait until all current
printk() messages (completed or in progress) have been printed.

Fixes: 3b604ca81202 ("printk: add pr_flush()")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240207134103.1357162-10-john.ogness@linutronix.de
Signed-off-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/printk/printk.c | 2 +-
kernel/printk/printk_ringbuffer.c | 105 ++++++++++++++++++++++++++++++
kernel/printk/printk_ringbuffer.h | 1 +
3 files changed, 107 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f2444b581e16c..d9420207282ac 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3761,7 +3761,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre

might_sleep();

- seq = prb_next_seq(prb);
+ seq = prb_next_reserve_seq(prb);

/* Flush the consoles so that records up to @seq are printed. */
console_lock();
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 4ce1826dc9426..d152b6bd35c9a 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -1974,6 +1974,111 @@ static u64 prb_first_seq(struct printk_ringbuffer *rb)
return seq;
}

+/**
+ * prb_next_reserve_seq() - Get the sequence number after the most recently
+ * reserved record.
+ *
+ * @rb: The ringbuffer to get the sequence number from.
+ *
+ * This is the public function available to readers to see what sequence
+ * number will be assigned to the next reserved record.
+ *
+ * Note that depending on the situation, this value can be equal to or
+ * higher than the sequence number returned by prb_next_seq().
+ *
+ * Context: Any context.
+ * Return: The sequence number that will be assigned to the next record
+ * reserved.
+ */
+u64 prb_next_reserve_seq(struct printk_ringbuffer *rb)
+{
+ struct prb_desc_ring *desc_ring = &rb->desc_ring;
+ unsigned long last_finalized_id;
+ atomic_long_t *state_var;
+ u64 last_finalized_seq;
+ unsigned long head_id;
+ struct prb_desc desc;
+ unsigned long diff;
+ struct prb_desc *d;
+ int err;
+
+ /*
+ * It may not be possible to read a sequence number for @head_id.
+ * So the ID of @last_finailzed_seq is used to calculate what the
+ * sequence number of @head_id will be.
+ */
+
+try_again:
+ last_finalized_seq = desc_last_finalized_seq(rb);
+
+ /*
+ * @head_id is loaded after @last_finalized_seq to ensure that
+ * it points to the record with @last_finalized_seq or newer.
+ *
+ * Memory barrier involvement:
+ *
+ * If desc_last_finalized_seq:A reads from
+ * desc_update_last_finalized:A, then
+ * prb_next_reserve_seq:A reads from desc_reserve:D.
+ *
+ * Relies on:
+ *
+ * RELEASE from desc_reserve:D to desc_update_last_finalized:A
+ * matching
+ * ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A
+ *
+ * Note: desc_reserve:D 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
+ * desc_read:C, which implies desc_reserve:D can be seen.
+ */
+ head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */
+
+ d = to_desc(desc_ring, last_finalized_seq);
+ state_var = &d->state_var;
+
+ /* Extract the ID, used to specify the descriptor to read. */
+ last_finalized_id = DESC_ID(atomic_long_read(state_var));
+
+ /* Ensure @last_finalized_id is correct. */
+ err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc);
+
+ if (err == -EINVAL) {
+ if (last_finalized_seq == 0) {
+ /*
+ * No record has been finalized or even reserved yet.
+ *
+ * The @head_id is initialized such that the first
+ * increment will yield the first record (seq=0).
+ * Handle it separately to avoid a negative @diff
+ * below.
+ */
+ if (head_id == DESC0_ID(desc_ring->count_bits))
+ return 0;
+
+ /*
+ * One or more descriptors are already reserved. Use
+ * the descriptor ID of the first one (@seq=0) for
+ * the @diff below.
+ */
+ last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1;
+ } else {
+ /* Record must have been overwritten. Try again. */
+ goto try_again;
+ }
+ }
+
+ /* Diff of known descriptor IDs to compute related sequence numbers. */
+ diff = head_id - last_finalized_id;
+
+ /*
+ * @head_id points to the most recently reserved record, but this
+ * function returns the sequence number that will be assigned to the
+ * next (not yet reserved) record. Thus +1 is needed.
+ */
+ return (last_finalized_seq + diff + 1);
+}
+
/*
* Non-blocking read of a record. Updates @seq to the last finalized record
* (which may have no data available).
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 70457916d577d..5aebe97bd4afc 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -380,6 +380,7 @@ bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,

u64 prb_first_valid_seq(struct printk_ringbuffer *rb);
u64 prb_next_seq(struct printk_ringbuffer *rb);
+u64 prb_next_reserve_seq(struct printk_ringbuffer *rb);

#ifdef CONFIG_64BIT

--
2.43.0