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

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Message-ID: <6ff2e489-7289-4840-868e-9401f26033c6@amd.com>
Date: Fri, 7 Mar 2025 11:54:56 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Hillf Danton <hdanton@...a.com>, Oleg Nesterov <oleg@...hat.com>
CC: 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

Hello Hiilf,

On 3/7/2025 11:38 AM, Hillf Danton wrote:
> 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

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