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

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Message-ID: <20250310124341.GB26382@redhat.com>
Date: Mon, 10 Mar 2025 13:43:42 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Hillf Danton <hdanton@...a.com>
Cc: K Prateek Nayak <kprateek.nayak@....com>,
	Mateusz Guzik <mjguzik@...il.com>,
	"Sapkal, Swapnil" <swapnil.sapkal@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] pipe_read: don't wake up the writer if the pipe is still
 full

On 03/10, Hillf Danton wrote:
>
> On Mon, 10 Mar 2025 12:09:15 +0100 Oleg Nesterov
> > On 03/10, Hillf Danton wrote:
> > > On Sun, 9 Mar 2025 18:02:55 +0100 Oleg Nesterov
> > > >
> > > > 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.
> >
> >     381          if (wake_writer)
> > ==> 382                  wake_up_interruptible_sync_poll(&pipe->wr_wait, EPOLLOUT | EPOLLWRNORM);
> >     383          if (wake_next_reader)
> >     384                  wake_up_interruptible_sync_poll(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
> >     385          kill_fasync(&pipe->fasync_writers, SIGIO, POLL_OUT);
> >     386          if (ret > 0)
> >     387                  file_accessed(filp);
> >     388          return ret;
> >
> > line 382, no?
> >
> Yes, but how is the wait loop at line-370 broken?
>
>  360                 }
>  361                 mutex_unlock(&pipe->mutex);
>  362
>  363                 BUG_ON(wake_writer);
>  364                 /*
>  365                  * But because we didn't read anything, at this point we can
>  366                  * just return directly with -ERESTARTSYS if we're interrupted,
>  367                  * since we've done any required wakeups and there's no need
>  368                  * to mark anything accessed. And we've dropped the lock.
>  369                  */
>  370                 if (wait_event_interruptible_exclusive(pipe->rd_wait, pipe_readable(pipe)) < 0)
>  371                         return -ERESTARTSYS;

Hmm. I don't understand you, again.

OK, once some writer writes at least one byte (this will make the
pipe_empty() condition false) and wakes this reader up.

If you meant something else, say, if you referred to you previous
scenario, please clarify your question.

Oleg.