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Subject: Re: [boost] [thread] Request review of new synchronisation object, boost::permit<>
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2014-05-05 14:55:19

On 5 May 2014 at 20:41, Peter Dimov wrote:

> POSIX says
> "It shall be safe to destroy an initialized condition variable upon which no
> threads are currently blocked."
> No threads are currently blocked upon the condition variable in your
> example. C++11 says the same:
> "~condition_variable();
> Requires: There shall be no thread blocked on *this."
> It is true that the POSIX standard supplies an example in which the thread
> calling pthread_cond_broadcast destroys the condvar, and it's true that
> C++11 addresses this same case in the note, but I think that the normative
> text clearly applies to your case too.

Ok, I'll repair the docs appropriately. Thanks for the info.

> > The way I came at it is this: examine this reduced code first:
> >
> > atomic<bool> &lock;
> > int result=0;
> > while(!cas_lock(lock));
> > if(!result) // can the compiler assume this is always false?
> >
> > Sure, the CAS lock prevents reordering of reads and writes either side of
> > the lock. But does it tell the compiler that it cannot assume that a
> > *local* variable's value will not have changed across the CAS lock?
> The compiler is within its rights to assume here that 'result' doesn't
> change, because it can prove that its address is not available to another
> thread. This is not the case in your example. The address of 'result' is -
> quite obviously - available to another thread, so a compiler that could
> prove that it isn't would be wrong.

But you've merely constructed the lambda, not executed it. The
compiler is surely right to think that without it being executed that
result's value will be constant?

> > Anyway, I still stick to my guns on that predicate checked values in wait
> > conditions really ought to be stored atomic to prevent reordering and
> > elision.
> The classic mutex/cv pattern does provide the necessary guarantees. Using
> atomics makes it easier for the programmer to write incorrect code.

I never claimed you shouldn't be using mutexes AND atomics. Condition
variables should always be used with mutexes, with no exception.


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