[ltt-dev] [URCU RFC patch 2/3] call_rcu: use futex for wakeup scheme

[compudj@krystal.dyndns.org (Mathieu Desnoyers)]

* Paolo Bonzini (pbonzini at redhat.com) wrote:
> On 06/06/2011 07:46 PM, Mathieu Desnoyers wrote:
>> +static void call_rcu_wait(struct call_rcu_data *crdp)
>> +{
>> +	/* Read call_rcu list before read futex */
>> +	cmm_smp_mb();
>> +	if (uatomic_read(&crdp->futex) == -1)
>> +		futex_async(&crdp->futex, FUTEX_WAIT, -1,
>> +		      NULL, NULL, 0);
>> +}
>> +
>> +static void call_rcu_wake_up(struct call_rcu_data *crdp)
>> +{
>> +	/* Write to call_rcu list before reading/writing futex */
>> +	cmm_smp_mb();
>> +	if (unlikely(uatomic_read(&crdp->futex) == -1)) {
>> +		uatomic_set(&crdp->futex, 0);
>> +		futex_async(&crdp->futex, FUTEX_WAKE, 1,
>> +		      NULL, NULL, 0);
>> +	}
>> +}
>
> I don't see crdp->futex ever set to -1, am I missing something obvious?


Good catch! I missed those parts from the urcu.c wait/wakeup scheme
(decrement initially, and also set to 0 explicitely when breaking the
loop without waiting). I just added them with commit
c768e45ed336970a42e58e679804f0f455422cd8

Thanks!

Mathieu

>
> Paolo
>
>
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-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com