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DateMon, 21 May 2007 21:55:40 +0200
FromEric Dumazet <>
SubjectRe: second, bigger problem with private futexes
Ulrich Drepper a écrit :
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> Eric Dumazet wrote:
>> 3) if condvar is PRIVATE and mutex is SHARED, a FUTEX_WAKE_PRIVATE
>> should be done. (and loose the REQUEUE optim)
>> Yes we could add a special futex primitive for this special case. But I
>> cannot see how a program could use such a construct.
>
> Very easily: mutexes are often created independently from the condvars
> and they are used for many things. Maybe a program is even creating all
> mutexes as shared to be ready for all situations. Normally doing this
> is no big problem, the performance penalties are minimal.
>
>
>> 4) if condvar is SHARED and mutex is private, we have a *problem*,
>> because the process doing the broadcast() can be in another mm. So a
>> requeue is not possible at all.
>
> It is if we can specify the owner of the mutex. I.e., the PID.

well, then we have a refcounting issue on pid , or problem in pid reuse.

>
> But yes, this case is extremely ugly.
>
>
> The problem is that all these cases worked nice so far. They all had
> the same good performance. Now we are severely penalizing code which
> mismatches condvar and mutex shared attributes. There is a good reason
> why we introduced FUTEX_CMP_REQUEUE, the benefits in certain programs is
> huge.

This analysis seems unfair to me, after a quick reading of glibc code.

Right now, glibc cannot use FUTEX_CMP_REQUEUE if condvar is pshared.

/* Do not use requeue for pshared condvars. */
if (cond->__data.__mutex == (void *) ~0l)
goto wake_all;
So how introducing private futexes is penalizing this case ?

Fact is that if condvar is pshared, we *cannot* use CMP_REQUEUE since threads
doing the broadcast() can be in a separate process and virtual address of
mutex of waiting threads could point to unrelated memory.

Thanks

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