Messages in this thread | | | Date | Thu, 13 Aug 2020 13:48:05 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH] task_work: only grab task signal lock when needed |
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On 08/12, Jens Axboe wrote: > > On 8/12/20 8:54 AM, Oleg Nesterov wrote: > > > > --- x/kernel/signal.c > > +++ x/kernel/signal.c > > @@ -2541,7 +2541,7 @@ bool get_signal(struct ksignal *ksig) > > > > relock: > > spin_lock_irq(&sighand->siglock); > > - current->jobctl &= ~JOBCTL_TASK_WORK; > > + smp_store_mb(current->jobctl, current->jobctl & ~JOBCTL_TASK_WORK); > > if (unlikely(current->task_works)) { > > spin_unlock_irq(&sighand->siglock); > > task_work_run(); > > > > I think this should work when paired with the READ_ONCE() on the > task_work_add() side.
It pairs with mb (implied by cmpxchg) before READ_ONCE. So we roughly have
task_work_add: get_signal:
STORE(task->task_works, new_work); STORE(task->jobctl); mb(); mb(); LOAD(task->jobctl); LOAD(task->task_works);
and we can rely on STORE-MB-LOAD.
> I haven't managed to reproduce badness with the > existing one that doesn't have the smp_store_mb() here, so can't verify > much beyond that...
Yes, the race is very unlikely. And the problem is minor, the target task can miss the new work added by TWA_SIGNAL and return from get_signal() without TIF_SIGPENDING.
> Are you going to send this out as a complete patch?
Jens, could you please send the patch? I am on vacation and travelling. Feel free to add my ACK.
Oleg.
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