On Thu, 29 Nov 2018 at 21:45, Peter Zijlstra <[email protected]> wrote: > > On Thu, Nov 29, 2018 at 02:12:32PM +0100, Peter Zijlstra wrote: > > > > +Cc davidlohr and waiman > > > Urgh; so the case where the cmpxchg() fails because it already has a > > wakeup in progress, which then 'violates' our expectation of when the > > wakeup happens. > > > > Yes, I think this is real, and worse, I think we need to go audit all > > wake_q_add() users and document this behaviour. > > > > In the ideal case we'd delay the actual wakeup to the last wake_up_q(), > > but I don't think we can easily fix that. > > See commit: 1d0dcb3ad9d3 ("futex: Implement lockless wakeups"), I think > that introduces the exact same bug. >
Hmm...Yes, even the thread may be in futex's wake_q and lead to rwsem's wakeup missing. Seems like fix this problem casy by case and document the behaviour is easier than delay the actual wakeup to the last wake_up_q()... Thanks, Yongji > Something like the below perhaps, altough this pattern seems to want a > wake_a_add() variant that already assumes get_task_struct(). > > diff --git a/kernel/futex.c b/kernel/futex.c > index f423f9b6577e..d14971f6ed3d 100644 > --- a/kernel/futex.c > +++ b/kernel/futex.c > @@ -1387,11 +1387,7 @@ static void mark_wake_futex(struct wake_q_head > *wake_q, struct futex_q *q) > if (WARN(q->pi_state || q->rt_waiter, "refusing to wake PI futex\n")) > return; > > - /* > - * Queue the task for later wakeup for after we've released > - * the hb->lock. wake_q_add() grabs reference to p. > - */ > - wake_q_add(wake_q, p); > + get_task_struct(p); > __unqueue_futex(q); > /* > * The waiting task can free the futex_q as soon as q->lock_ptr = NULL > @@ -1401,6 +1397,13 @@ static void mark_wake_futex(struct wake_q_head > *wake_q, struct futex_q *q) > * plist_del in __unqueue_futex(). > */ > smp_store_release(&q->lock_ptr, NULL); > + > + /* > + * Queue the task for later wakeup for after we've released > + * the hb->lock. wake_q_add() grabs reference to p. > + */ > + wake_q_add(wake_q, p); > + put_task_struct(p); > } > > /*
