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pipe: fix and clarify pipe read wakeup logic 

This is the read side version of the previous commit: it simplifies the
logic to only wake up waiting writers when necessary, and makes sure to
use a synchronous wakeup.  This time not so much for GNU make jobserver
reasons (that pipe never fills up), but simply to get the writer going
quickly again.

A bit less verbose commentary this time, if only because I assume that
the write side commentary isn't going to be ignored if you touch this
code.

Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Linus Torvalds 2019-12-07 12:54:26 -08:00
parent 1b6b26ae70
commit f467a6a664
1 changed files with 18 additions and 13 deletions
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31
fs/pipe.c
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@ -276,16 +276,25 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
size_t total_len = iov_iter_count(to);
struct file *filp = iocb->ki_filp;
struct pipe_inode_info *pipe = filp->private_data;
int do_wakeup;
bool was_full;
ssize_t ret;
/* Null read succeeds. */
if (unlikely(total_len == 0))
return 0;
do_wakeup = 0;
ret = 0;
__pipe_lock(pipe);
/*
* We only wake up writers if the pipe was full when we started
* reading in order to avoid unnecessary wakeups.
*
* But when we do wake up writers, we do so using a sync wakeup
* (WF_SYNC), because we want them to get going and generate more
* data for us.
*/
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
for (;;) {
unsigned int head = pipe->head;
unsigned int tail = pipe->tail;
@ -324,19 +333,11 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
}
if (!buf->len) {
bool wake;
pipe_buf_release(pipe, buf);
spin_lock_irq(&pipe->wait.lock);
tail++;
pipe->tail = tail;
do_wakeup = 1;
wake = head - (tail - 1) == pipe->max_usage / 2;
if (wake)
wake_up_locked_poll(
&pipe->wait, EPOLLOUT | EPOLLWRNORM);
spin_unlock_irq(&pipe->wait.lock);
if (wake)
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
total_len -= chars;
if (!total_len)
@ -365,13 +366,17 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
ret = -ERESTARTSYS;
break;
}
if (was_full) {
wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
pipe_wait(pipe);
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
}
__pipe_unlock(pipe);
/* Signal writers asynchronously that there is more room. */
if (do_wakeup) {
wake_up_interruptible_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
if (was_full) {
wake_up_interruptible_sync_poll(&pipe->wait, EPOLLOUT | EPOLLWRNORM);
kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
}
if (ret > 0)