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

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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@ionos.com>
Message-Id: <20230921075755.1378787-4-max.kellermann@ionos.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
Max Kellermann 2023-09-21 09:57:55 +02:00 committed by Christian Brauner
parent dfaabf916b
commit 478dbf1217
No known key found for this signature in database
GPG Key ID: 91C61BC06578DCA2
1 changed files with 32 additions and 11 deletions
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43
fs/pipe.c
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@ -227,6 +227,36 @@ static inline bool pipe_readable(const struct pipe_inode_info *pipe)
return !pipe_empty(head, tail) || !writers;
}
static inline unsigned int pipe_update_tail(struct pipe_inode_info *pipe,
struct pipe_buffer *buf,
unsigned int tail)
{
pipe_buf_release(pipe, buf);
/*
* If the pipe has a watch_queue, we need additional protection
* by the spinlock because notifications get posted with only
* this spinlock, no mutex
*/
if (pipe_has_watch_queue(pipe)) {
spin_lock_irq(&pipe->rd_wait.lock);
#ifdef CONFIG_WATCH_QUEUE
if (buf->flags & PIPE_BUF_FLAG_LOSS)
pipe->note_loss = true;
#endif
pipe->tail = ++tail;
spin_unlock_irq(&pipe->rd_wait.lock);
return tail;
}
/*
* Without a watch_queue, we can simply increment the tail
* without the spinlock - the mutex is enough.
*/
pipe->tail = ++tail;
return tail;
}
static ssize_t
pipe_read(struct kiocb *iocb, struct iov_iter *to)
{
@ -320,17 +350,8 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
buf->len = 0;
}
if (!buf->len) {
pipe_buf_release(pipe, buf);
spin_lock_irq(&pipe->rd_wait.lock);
#ifdef CONFIG_WATCH_QUEUE
if (buf->flags & PIPE_BUF_FLAG_LOSS)
pipe->note_loss = true;
#endif
tail++;
pipe->tail = tail;
spin_unlock_irq(&pipe->rd_wait.lock);
}
if (!buf->len)
tail = pipe_update_tail(pipe, buf, tail);
total_len -= chars;
if (!total_len)
break; /* common path: read succeeded */