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
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
