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

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Message-ID: <20250307060827.3083-1-hdanton@sina.com>
Date: Fri,  7 Mar 2025 14:08:26 +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 Thu, 6 Mar 2025 10:30:21 +0100 Oleg Nesterov <oleg@...hat.com>
> On 03/06, Hillf Danton wrote:
> > On Wed, 5 Mar 2025 12:44:34 +0100 Oleg Nesterov <oleg@...hat.com>
> > > 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
	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