On Thu, May 09, 2019 at 06:19:25PM +0200, Sebastian Andrzej Siewior wrote: > Please: > - add some RT developers on Cc: > - add lkml > - use [PATCH RT] instead just [PATCH] so it is visible that you target > the RT tree.
Will do. I'll add your diagram below, too. > > On 2019-05-08 15:57:28 [-0500], [email protected] wrote: > > From: Corey Minyard <[email protected]> > > > > The function call do_wait_for_common() has a race condition that > > can result in lockups waiting for completions. Adding the thread > > to (and removing the thread from) the wait queue for the completion > > is done outside the do loop in that function. However, if the thread > > is woken up, the swake_up_locked() function will delete the entry > > from the wait queue. If that happens and another thread sneaks > > in and decrements the done count in the completion to zero, the > > loop will go around again, but the thread will no longer be in the > > wait queue, so there is no way to wake it up. > > > > Fix it by adding/removing the thread to/from the wait queue inside > > the do loop. > > So you are saying: > T0 T1 T2 > wait_for_completion() > do_wait_for_common() > __prepare_to_swait() > schedule() > complete() > x->done++ (0 -> 1) > raw_spin_lock_irqsave() > swake_up_locked() > wait_for_completion() > wake_up_process(T0) > list_del_init() > raw_spin_unlock_irqrestore() > > raw_spin_lock_irq(&x->wait.lock) > raw_spin_lock_irq(&x->wait.lock) x->done != > UINT_MAX, 1 -> 0 > return 1 > > raw_spin_unlock_irq(&x->wait.lock) > while (!x->done && timeout), > continue loop, not enqueued > on &x->wait > > The difference compared to the non-swait based implementation is that > swake_up_locked() removes woken up tasks from the list while the other > implementation (wait_queue_entry based, default_wake_function()) does > not. Buh Yes, exactly. I was wondering if swait could be changed to not remove the waiter, but that seemed like a bad idea. It is an unusual semantic, though. I thought some more about this, wondering why everything isn't keeling over because of this. I'm guessing that just about everything using completions has a single waiter, so it doesn't matter. I just wrote some code that has a bunch of waiters, so I hit it. -corey > > One question for the upstream completion implementation: > completion_done() returns true if there are no waiters. It acquires the > wait.lock to ensure that complete()/complete_all() is done. However, > once complete releases the lock it is guaranteed that the wake_up() (for > the waiter) occurred. The waiter task still needs to be remove itself > from the wait-queue before the completion can be removed. > Do I miss something? > > > Fixes: a04ff6b4ec4ee7e ("completion: Use simple wait queues") > > Signed-off-by: Corey Minyard <[email protected]> > > --- > > I sent the wrong version of this, I had spotted this before but didn't > > fix it here. Adding the thread to the wait queue needs to come after > > the signal check. Sorry about the noise. > > > > kernel/sched/completion.c | 8 ++++---- > > 1 file changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c > > index 755a58084978..4f9b4cc0c95a 100644 > > --- a/kernel/sched/completion.c > > +++ b/kernel/sched/completion.c > > @@ -70,20 +70,20 @@ do_wait_for_common(struct completion *x, > > long (*action)(long), long timeout, int state) > > { > > if (!x->done) { > > - DECLARE_SWAITQUEUE(wait); > > - > > - __prepare_to_swait(&x->wait, &wait); > > you can keep DECLARE_SWAITQUEUE remove just __prepare_to_swait() > > > do { > > + DECLARE_SWAITQUEUE(wait); > > + > > if (signal_pending_state(state, current)) { > > timeout = -ERESTARTSYS; > > break; > > } > > + __prepare_to_swait(&x->wait, &wait); > > add this, yes and you are done. > > > __set_current_state(state); > > raw_spin_unlock_irq(&x->wait.lock); > > timeout = action(timeout); > > raw_spin_lock_irq(&x->wait.lock); > > + __finish_swait(&x->wait, &wait); > > } while (!x->done && timeout); > > - __finish_swait(&x->wait, &wait); > > if (!x->done) > > return timeout; > > } > > Sebastian
