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Groups > linux.kernel > #1526627
| From | Oleg Nesterov <oleg@redhat.com> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups |
| Date | 2016-11-21 13:30 +0100 |
| Message-ID | <sFVTr-18L-7@gated-at.bofh.it> (permalink) |
| References | <sEWye-2tW-9@gated-at.bofh.it> <sEWyf-2tW-55@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
On 11/18, Davidlohr Bueso wrote:
>
> +static bool __readers_active_check(struct percpu_rw_semaphore *sem)
> +{
> + return !(per_cpu_sum(*sem->read_count) !=0);
> +}
Hmm,
return per_cpu_sum(*sem->read_count) == 0;
looks more clear, but this is minor,
> int __percpu_init_rwsem(struct percpu_rw_semaphore *sem,
> const char *name, struct lock_class_key *rwsem_key)
> {
> @@ -103,41 +141,11 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
> __this_cpu_dec(*sem->read_count);
>
> /* Prod writer to recheck readers_active */
> - swake_up(&sem->writer);
> + if (__readers_active_check(sem))
> + swake_up(&sem->writer);
Suppose we have 2 active readers which call __percpu_up_read() at the same
time and the pending writer sleeps.
What guarantees that one of these readers will observe per_cpu_sum() == 0 ?
They both can read the old value of the remote per-cpu counter, no?
Oleg.
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[PATCH -tip 0/3] locking/percpu-rwsem: writer-side optimizations Davidlohr Bueso <dave@stgolabs.net> - 2016-11-18 20:00 +0100
[PATCH 1/3] locking/percpu-rwsem: Move text file into Documentation/locking/ Davidlohr Bueso <dave@stgolabs.net> - 2016-11-18 20:00 +0100
[PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Davidlohr Bueso <dave@stgolabs.net> - 2016-11-18 20:00 +0100
Re: [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Oleg Nesterov <oleg@redhat.com> - 2016-11-21 14:00 +0100
Re: [PATCH 2/3] locking/percpu-rwsem: Replace bulky wait-queues with swait Davidlohr Bueso <dave@stgolabs.net> - 2016-11-21 18:30 +0100
[PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Davidlohr Bueso <dave@stgolabs.net> - 2016-11-18 20:00 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Peter Zijlstra <peterz@infradead.org> - 2016-11-21 13:30 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Oleg Nesterov <oleg@redhat.com> - 2016-11-21 13:50 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Oleg Nesterov <oleg@redhat.com> - 2016-11-21 16:10 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Davidlohr Bueso <dave@stgolabs.net> - 2016-11-22 05:00 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Oleg Nesterov <oleg@redhat.com> - 2016-11-23 15:50 +0100
Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups Oleg Nesterov <oleg@redhat.com> - 2016-11-21 13:30 +0100
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