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Subject: Re: [Boost-users] [thread] release lock before cond.var. notified?
From: Gavin Lambert (gavinl_at_[hidden])
Date: 2013-09-09 18:44:47

On 9/09/2013 10:34 p.m., Quoth Thomas M:
> Now the question: Is there a strong point pro or against releasing
> controllerLock before the notify_all-signal is sent?
> E.g. is there preference for the first or second variant as below (see
> placement of curly brackets!), or even a need for one to ensure
> correctness?

The normal problem, IIRC, is that notify_all() will only wake up those
threads that are actually sleeping on the condition variable already.
If you release the lock early, then another thread might enter the
locked region, discover that the condition is false, and be about to
sleep on the condition variable when the notify is triggered -- it then
goes to sleep permanently because it missed the wakeup event.

> // simplified code snippet begin - Variant 2
> // notify_all is issued when controllerLock has already been released
> {
> boost::unique_lock<boost::mutex> controllerLock(someMutex);
> Continue = true;
> }
> someCondVar.notify_all();
> // simplified code snippet end

Given that you are updating the condition within the mutex I believe
that this is safe from the above issue. However it still may not be
safe due to other factors -- for instance, if the memory containing
someCondVar becomes eligible for deletion once the condition is true
(which happens more often than you'd think, particularly when the
condition includes some sort of termination flag/event) then variant 1
is still safe while variant 2 is not.

In terms of efficiency, variant 2 is probably more efficient than
variant 1 on a uniprocessor system, but on a multicore system they're
probably about the same. (As on uniprocessor there is a chance that the
notify could cause an immediate preemption, lockout, and switchback with
variant 1, while on a multicore the notifying thread should be able to
continue to release the lock.)

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