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

[Hillf Danton]

On Sun, 9 Mar 2025 18:02:55 +0100 Oleg Nesterov
> 
> Well. Prateek has already provide the lengthy/thorough explanation,
> but let me add anyway...
> 
lengthy != correct

> On 03/08, Hillf Danton wrote:
> > On Fri, 7 Mar 2025 13:34:43 +0100 Oleg Nesterov <oleg@xxxxxxxxxx>
> > > 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@xxxxxxx>
> > > > > >> 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@xxxxxxxxxx/
> ...
> > --- 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).
> 
Given never and the BUG_ON below, you accidentally prove that Prateek's
comment is false, no?

> 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.
> 
Fine, before checking my scenario once more, feel free to pinpoint the
line number where writer is woken up, with the change below applied.

> 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,
> 
>