Commit f467a6a6 authored by Linus Torvalds's avatar Linus Torvalds
Browse files

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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 1b6b26ae

fs/pipe.c

+18 −13
Original line number Diff line number Diff line
@@ -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)

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