Hi Hillf,
again, I am not sure we understand each other, at least me...
On 03/05, Hillf Danton wrote:
>
> On Wed, 5 Mar 2025 00:49:09 +0100 Oleg Nesterov <oleg@xxxxxxxxxx>
> >
> > Of course! Again, whatever the woken writer checks in pipe_writable()
> > lockless, another writer can make pipe_full() true again.
> >
> > But why do we care? Why do you think that the change you propose makes
>
> Because of the hang reported.
The hang happened because pipe_writable() could wrongly return false
when the buffer is not full.
Afaics, the Prateek's or Linus's fix solve this problem, and this is
all we need.
> > more sense than the fix from Prateek or the (already merged) Linus's fix?
> >
> See the loop in ___wait_event(),
>
> for (;;) {
> prepare_to_wait_event();
>
> // flip
> if (condition)
> break;
>
> schedule();
> }
I will assume that this "// flip" means the case I described above:
before this writer checks the condition, another writer comes and
increments pipe->head.
> 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.
Even if flips again and becomes false right after the writer called
pipe_writable().
Note that it checks the condition after set_current_state(INTERRUPTIBLE)
and it is still on the pipe->wr_wait->head list.
The 2nd "flip" is only possible if some reader flushes another buffer
and updates pipe->tail, and in this case it will wake this writer again.
So I still can't understand your concerns, sorry.
Oleg.
