Messages in this thread |  | | | Date | Tue, 7 Jun 2011 22:36:13 +0800 | | Subject | Re: [PATCH] sched: correct testing need_resched in mutex_spin_on_owner() | | From | Hillf Danton <> |
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On Tue, Jun 7, 2011 at 10:24 PM, Peter Zijlstra <peterz@infradead.org> wrote: > On Tue, 2011-06-07 at 22:10 +0800, Hillf Danton wrote: >> On Tue, Jun 7, 2011 at 9:47 PM, Peter Zijlstra <peterz@infradead.org> wrote: >> > On Tue, 2011-06-07 at 21:41 +0800, Hillf Danton wrote: >> >> It is suppose to check the owner task that is not absolutly running on the >> >> local CPU, >> > >> > Oh, why do you think so? >> > >> as the comment in __mutex_lock_common() says, >> >> #ifdef CONFIG_MUTEX_SPIN_ON_OWNER >> /* >> * Optimistic spinning. >> * >> * We try to spin for acquisition when we find that there are no >> * pending waiters and the lock owner is currently running on a >> *************************** >> * (different) CPU. >> *************************** >> * ..... >> */ > > Which is exactly what owner_running() does, clearly it cannot run on the > current cpu, since then we wouldn't be running to check things, so for > as long as owner is on_cpu we spin. > > However, > >> need_resched is checked after true is returned by owner_running(), >> in other words, owner is still on its CPU, so owner should be check >> here. Even ower's CPU == this CPU, checking owner also gives >> correct result. > > no, after rcu_read_unlock() in owner_running() -- read that comment > again -- the owner pointer can be free or reused memory. > > Also, since we already check owner_running() this need_resched() is > clearly something different. By your argument it would be superfluous > not wrong. > > Now since it appears superfluous, ask yourself why it would exist. The > answer is simple, we should not be greedy and consume our cpu when > there's someone else that wants to run, we should yield the cpu on > need_resched(), which is exactly what happens. > If you are right, the following comment also in __mutex_lock_common()
for (;;) { struct task_struct *owner; /* * If there's an owner, wait for it to either * release the lock or go to sleep. */ owner = ACCESS_ONCE(lock->owner); if (owner && !mutex_spin_on_owner(lock, owner)) break; looks misleading too, but if owner is on this CPU, for what does we wait?
thanks Hillf -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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