Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still full

[Oleg Nesterov <oleg@redhat.com>]

Well. Prateek has already provide the lengthy/thorough explanation,
but let me add anyway...

On 03/08, Hillf Danton wrote:
>
> On Fri, 7 Mar 2025 13:34:43 +0100 Oleg Nesterov <oleg@redhat.com>
> > On 03/07, Oleg Nesterov wrote:
> > > On 03/07, Hillf Danton wrote:
> > > > On Fri, 7 Mar 2025 11:54:56 +0530 K Prateek Nayak <kprateek.nayak@amd.com>
> > > > >> step-03
> > > > >> 	task-118766 new reader
> > > > >> 	makes pipe empty
> > > > >
> > > > >Reader seeing a pipe full should wake up a writer allowing 118768 to
> > > > >wakeup again and fill the pipe. Am I missing something?
> > > > >
> > > > Good catch, but that wakeup was cut off [2,3]
> >
> > Please note that "that wakeup" was _not_ removed by the patch below.
> >
> After another look, you did cut it.

I still don't think so.

> Link: https://lore.kernel.org/all/20250209150718.GA17013@redhat.com/
...
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -360,29 +360,9 @@ anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
>  			break;
>  		}
>  		mutex_unlock(&pipe->mutex);
> -
>  		/*
>  		 * We only get here if we didn't actually read anything.
>  		 *
> -		 * However, we could have seen (and removed) a zero-sized
> -		 * pipe buffer, and might have made space in the buffers
> -		 * that way.
> -		 *
> -		 * You can't make zero-sized pipe buffers by doing an empty
> -		 * write (not even in packet mode), but they can happen if
> -		 * the writer gets an EFAULT when trying to fill a buffer
> -		 * that already got allocated and inserted in the buffer
> -		 * array.
> -		 *
> -		 * So we still need to wake up any pending writers in the
> -		 * _very_ unlikely case that the pipe was full, but we got
> -		 * no data.
> -		 */
> -		if (unlikely(wake_writer))
> -			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
> -		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> -
> -		/*
>  		 * But because we didn't read anything, at this point we can
>  		 * just return directly with -ERESTARTSYS if we're interrupted,
>  		 * since we've done any required wakeups and there's no need
> @@ -391,7 +371,6 @@ anon_pipe_read(struct kiocb *iocb, struct iov_iter *to)
>  		if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
>  			return -ERESTARTSYS;
>
> -		wake_writer = false;
>  		wake_next_reader = true;
>  		mutex_lock(&pipe->mutex);
>  	}

Please note that in this particular case (hackbench testing)
pipe_write() -> copy_page_from_iter() never fails. So wake_writer is
never true before pipe_reader() calls wait_event(pipe->rd_wait).

So (again, in this particular case) we could apply the patch below
on top of Linus's tree.

So, with or without these changes, the writer should be woken up at
step-03 in your scenario.

Oleg.
---

--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -360,27 +360,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
 		}
 		mutex_unlock(&pipe->mutex);
 
-		/*
-		 * We only get here if we didn't actually read anything.
-		 *
-		 * However, we could have seen (and removed) a zero-sized
-		 * pipe buffer, and might have made space in the buffers
-		 * that way.
-		 *
-		 * You can't make zero-sized pipe buffers by doing an empty
-		 * write (not even in packet mode), but they can happen if
-		 * the writer gets an EFAULT when trying to fill a buffer
-		 * that already got allocated and inserted in the buffer
-		 * array.
-		 *
-		 * So we still need to wake up any pending writers in the
-		 * _very_ unlikely case that the pipe was full, but we got
-		 * no data.
-		 */
-		if (unlikely(wake_writer))
-			wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
-		kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
-
+		BUG_ON(wake_writer);
 		/*
 		 * But because we didn't read anything, at this point we can
 		 * just return directly with -ERESTARTSYS if we're interrupted,