Messages in this thread Patch in this message | | | Date | Mon, 21 Nov 2016 16:07:23 +0100 | From | Oleg Nesterov <> | Subject | Re: [PATCH 3/3] locking/percpu-rwsem: Avoid unnecessary writer wakeups |
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On 11/21, Oleg Nesterov wrote: > > No, no, I meant that afaics both readers can see per_cpu_sum() != 0 and > thus the writer won't be woken up. Till the next down_read/up_read. > > Suppose that we have 2 CPU's, both counters == 1, both readers decrement. > its counter at the same time. > > READER_ON_CPU_0 READER_ON_CPU_1 > > --ctr_0; --ctr_1; > > if (ctr_0 + ctr_1) if (ctr_0 + ctr_1) > wakeup(); wakeup(); > > Why we can't miss a wakeup? > > This patch doesn't even add a barrier, but I think wmb() won't be enough > anyway.
And in fact I am not sure this optimization makes sense... But it would be nice to avoid wake_up() when the writer sleeps in rcu_sync_enter(). Or this is the "slow mode" sem (cgroup_threadgroup_rwsem).
I need to re-check, but what do you think about the change below?
Oleg.
--- x/kernel/locking/percpu-rwsem.c +++ x/kernel/locking/percpu-rwsem.c @@ -103,7 +103,9 @@ void __percpu_up_read(struct percpu_rw_s __this_cpu_dec(*sem->read_count); /* Prod writer to recheck readers_active */ - wake_up(&sem->writer); + smp_mb(); + if (sem->readers_block) + wake_up(&sem->writer); } EXPORT_SYMBOL_GPL(__percpu_up_read);
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