On Fri, 7 Mar 2025 11:54:56 +0530 K Prateek Nayak <kprateek.nayak@xxxxxxx>
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?
>
Good catch, but that wakeup was cut off [2,3]
[2] https://lore.kernel.org/lkml/20250304123457.GA25281@xxxxxxxxxx/
[3] https://lore.kernel.org/all/20250210114039.GA3588@xxxxxxxxxx/
>> 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
