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

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Message-ID: <20250307104654.3100-1-hdanton@sina.com>
Date: Fri,  7 Mar 2025 18:46:53 +0800
From: Hillf Danton <hdanton@...a.com>
To: K Prateek Nayak <kprateek.nayak@....com>
Cc: Hillf Danton <hdanton@...a.com>,
	Oleg Nesterov <oleg@...hat.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 Fri, 7 Mar 2025 11:54:56 +0530 K Prateek Nayak <kprateek.nayak@....com>
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?
>
Good catch, but that wakeup was cut off [2,3]

[2] https://lore.kernel.org/lkml/20250304123457.GA25281@redhat.com/
[3] https://lore.kernel.org/all/20250210114039.GA3588@redhat.com/

>> 	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