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Groups > linux.kernel > #1404718
| From | Peter Zijlstra <peterz@infradead.org> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: sem_lock() vs qspinlocks |
| Date | 2016-05-20 23:00 +0200 |
| Message-ID | <rAZGy-2cl-1@gated-at.bofh.it> (permalink) |
| References | <rALkd-1uD-9@gated-at.bofh.it> <rARfX-51X-9@gated-at.bofh.it> <rAZwS-27c-11@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On Fri, May 20, 2016 at 04:44:19PM -0400, Waiman Long wrote:
> On 05/20/2016 07:58 AM, Peter Zijlstra wrote:
> >On Thu, May 19, 2016 at 10:39:26PM -0700, Davidlohr Bueso wrote:
> >>As such, the following restores the behavior of the ticket locks and 'fixes'
> >>(or hides?) the bug in sems. Naturally incorrect approach:
> >>
> >>@@ -290,7 +290,8 @@ static void sem_wait_array(struct sem_array *sma)
> >>
> >> for (i = 0; i< sma->sem_nsems; i++) {
> >> sem = sma->sem_base + i;
> >>- spin_unlock_wait(&sem->lock);
> >>+ while (atomic_read(&sem->lock))
> >>+ cpu_relax();
> >> }
> >> ipc_smp_acquire__after_spin_is_unlocked();
> >>}
> >The actual bug is clear_pending_set_locked() not having acquire
> >semantics. And the above 'fixes' things because it will observe the old
> >pending bit or the locked bit, so it doesn't matter if the store
> >flipping them is delayed.
>
> The clear_pending_set_locked() is not the only place where the lock is set.
> If there are more than one waiter, the queuing patch will be used instead.
> The set_locked(), which is also an unordered store, will then be used to set
> the lock.
Ah yes. I didn't get that far. One case was enough :-)
Back to linux.kernel | Previous | Next — Previous in thread | Find similar | Unroll thread
sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 07:40 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:00 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 17:10 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:10 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 17:30 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:30 +0200
Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-20 22:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:00 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 03:00 +0200
Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-21 06:10 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-21 09:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:00 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:20 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:20 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 11:40 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 10:40 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 11:10 +0200
Re: sem_lock() vs qspinlocks Ingo Molnar <mingo@kernel.org> - 2016-05-20 12:10 +0200
Re: sem_lock() vs qspinlocks Mel Gorman <mgorman@techsingularity.net> - 2016-05-20 12:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 14:00 +0200
Re: sem_lock() vs qspinlocks Boqun Feng <boqun.feng@gmail.com> - 2016-05-20 16:10 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 17:30 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 18:10 +0200
Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-20 19:10 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:10 +0200
Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-20 23:50 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 02:50 +0200
Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-21 04:40 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-21 09:40 +0200
Re: sem_lock() vs qspinlocks Manfred Spraul <manfred@colorfullife.com> - 2016-05-21 15:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-24 13:00 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-21 19:20 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-23 14:30 +0200
Re: sem_lock() vs qspinlocks Linus Torvalds <torvalds@linux-foundation.org> - 2016-05-23 20:00 +0200
Re: sem_lock() vs qspinlocks Boqun Feng <boqun.feng@gmail.com> - 2016-05-25 08:40 +0200
Re: sem_lock() vs qspinlocks Manfred Spraul <manfred@colorfullife.com> - 2016-05-22 10:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-22 11:40 +0200
Re: sem_lock() vs qspinlocks Davidlohr Bueso <dave@stgolabs.net> - 2016-05-20 18:30 +0200
Re: sem_lock() vs qspinlocks Waiman Long <waiman.long@hpe.com> - 2016-05-20 22:50 +0200
Re: sem_lock() vs qspinlocks Peter Zijlstra <peterz@infradead.org> - 2016-05-20 23:00 +0200
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