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

[K Prateek Nayak]

Hello Hiilf,

On 3/7/2025 11:38 AM, Hillf Danton wrote:
> On Thu, 6 Mar 2025 10:30:21 +0100 Oleg Nesterov <oleg@xxxxxxxxxx>
> > On 03/06, Hillf Danton wrote:
> > > On Wed, 5 Mar 2025 12:44:34 +0100 Oleg Nesterov <oleg@xxxxxxxxxx>
> > > > On 03/05, Hillf Danton wrote:
> > > > > See the loop in  ___wait_event(),
> > > > >
> > > > > 	for (;;) {
> > > > > 		prepare_to_wait_event();
> > > > >
> > > > > 		// flip
> > > > > 		if (condition)
> > > > > 			break;
> > > > >
> > > > > 		schedule();
> > > > > 	}
> > > > >
> > > > > After wakeup, waiter will sleep again if condition flips false on the waker
> > > > > side before waiter checks condition, even if condition is atomic, no?
> > > >
> > > > Yes, but in this case pipe_full() == true is correct, this writer can
> > > > safely sleep.
> > > No, because no reader is woken up before sleep to make pipe not full.
> >
> > Why the reader should be woken before this writer sleeps? Why the reader
> > should be woken at all in this case (when pipe is full again) ?
> "to make pipe not full" failed to prevent you asking questions like this one.
>
> > We certainly can't understand each other.
> >
> > Could your picture the exact scenario/sequence which can hang?
> If you think the scenario in commit 3d252160b818 [1] is correct, check
> the following one.
>
> step-00
> 	pipe->head = 36
> 	pipe->tail = 36
> 	after 3d252160b818
>
> step-01
> 	task-118762 writer
> 	pipe->head++;
> 	wakes up task-118740 and task-118768
>
> step-02
> 	task-118768 writer
> 	makes pipe full;
> 	sleeps without waking up any reader as
> 	pipe was not empty after step-01
>
> 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?

> 	sleeps
>
> step-04
> 	task-118740 reader
> 	sleeps as pipe is empty
>
> [ Tasks 118740 and 118768 can then indefinitely wait on each other. ]
>
>
> [1] https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/fs/pipe.c?id=3d252160b818045f3a152b13756f6f37ca34639d

--
Thanks and Regards,
Prateek