[PATCH printk v2 6/9] printk: Wait for all reserved records with pr_flush()

[John Ogness <john.ogness@linutronix.de>]

Currently pr_flush() will only wait for records that were
available to readers at the time of the call. But there may be
more records (non-finalized) with 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.

Fixes: 3b604ca81202 ("printk: add pr_flush()")
Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
 kernel/printk/printk.c            |  2 +-
 kernel/printk/printk_ringbuffer.c | 96 +++++++++++++++++++++++++++++++
 kernel/printk/printk_ringbuffer.h |  1 +
 3 files changed, 98 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 82dc2c7949b7..f3a7f5a6f6f8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3755,7 +3755,7 @@ 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 7c121fd68330..dc83569d3a3a 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -2012,6 +2012,102 @@ 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(desc_ring);
+
+	/*
+	 * @head_id is loaded after @last_finalized_seq to ensure that it is
+	 * at or beyond @last_finalized_seq.
+	 *
+	 * 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) {
+			/*
+			 * @last_finalized_seq still contains its initial
+			 * value. Probably no record has been finalized yet.
+			 * This means the ringbuffer is not yet full and the
+			 * @head_id value can be used directly (subtracting
+			 * off its initial value).
+			 *
+			 * Because of hack#2 of the bootstrapping phase, the
+			 * @head_id initial value must be handled separately.
+			 */
+			if (head_id == DESC0_ID(desc_ring->count_bits))
+				return 0;
+
+			last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1;
+		} else {
+			/* Record must have been overwritten. Try again. */
+			goto try_again;
+		}
+	}
+
+	diff = head_id - last_finalized_id;
+
+	return (last_finalized_seq + diff);
+}
+
 /*
  * Non-blocking read of a record.
  *
diff --git a/kernel/printk/printk_ringbuffer.h b/kernel/printk/printk_ringbuffer.h
index 5c09061433f3..b48b44ecbe6d 100644
--- a/kernel/printk/printk_ringbuffer.h
+++ b/kernel/printk/printk_ringbuffer.h
@@ -394,5 +394,6 @@ 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);
 
 #endif /* _KERNEL_PRINTK_RINGBUFFER_H */
-- 
2.39.2