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

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Message-ID: <20250310113726.3266-1-hdanton@sina.com>
Date: Mon, 10 Mar 2025 19:37:24 +0800
From: Hillf Danton <hdanton@...a.com>
To: Oleg Nesterov <oleg@...hat.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 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;
 372
 373                 wake_writer = false;
 374                 wake_next_reader = true;
 375                 mutex_lock(&pipe->mutex);