[PATCH 4/4] fs/pipe: use spinlock in pipe_read() only if there is a watch_queue

[Max Kellermann]

If there is no watch_queue, holding the pipe mutex is enough to
prevent concurrent writes, and we can avoid the spinlock.

O_NOTIFICATION_QUEUE is an exotic and rarely used feature, and of all
the pipes that exist at any given time, only very few actually have a
watch_queue, therefore it appears worthwile to optimize the common
case.

This patch does not optimize pipe_resize_ring() where the spinlocks
could be avoided as well; that does not seem like a worthwile
optimization because this function is not called often.

Related commits:

- commit 8df441294dd3 ("pipe: Check for ring full inside of the
  spinlock in pipe_write()")
- commit b667b8673443 ("pipe: Advance tail pointer inside of wait
  spinlock in pipe_read()")
- commit 189b0ddc2451 ("pipe: Fix missing lock in pipe_resize_ring()")

Signed-off-by: Max Kellermann <max.kellermann@xxxxxxxxx>
---
 fs/pipe.c | 32 ++++++++++++++++++++++++++------
 1 file changed, 26 insertions(+), 6 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 939def02c18c..da557eff9560 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -322,14 +322,34 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 
 			if (!buf->len) {
 				pipe_buf_release(pipe, buf);
-				spin_lock_irq(&pipe->rd_wait.lock);
+
+				if (pipe_has_watch_queue(pipe)) {
+					/* if the pipe has a
+					 * watch_queue, we need
+					 * additional protection by
+					 * the spinlock because
+					 * notifications get posted
+					 * with only this spinlock, no
+					 * mutex
+					 */
+
+					spin_lock_irq(&pipe->rd_wait.lock);
 #ifdef CONFIG_WATCH_QUEUE
-				if (buf->flags & PIPE_BUF_FLAG_LOSS)
-					pipe->note_loss = true;
+					if (buf->flags & PIPE_BUF_FLAG_LOSS)
+						pipe->note_loss = true;
 #endif
-				tail++;
-				pipe->tail = tail;
-				spin_unlock_irq(&pipe->rd_wait.lock);
+					tail++;
+					pipe->tail = tail;
+					spin_unlock_irq(&pipe->rd_wait.lock);
+				} else {
+					/* without a watch_queue, we
+					 * can simply increment the
+					 * tail without the spinlock -
+					 * the mutex is enough
+					 */
+
+					pipe->tail = ++tail;
+				}
 			}
 			total_len -= chars;
 			if (!total_len)
-- 
2.39.2