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Subject: Re: [boost] [interprocess] locked mutex and process killed
From: Kim Barrett (kab.conundrums_at_[hidden])
Date: 2009-03-25 05:53:57

At 11:40 AM +0300 3/25/09, Dmitry Goncharov wrote:
>Even in this situation you can do something. After a thread (or a
>process) which holds a mutex locked gets killed any attempt to
>unlock the mutex
>will (and should) fail with EPERM.

Per POSIX, if the mutex type is PTHREAD_MUTEX_NORMAL, unlock by
non-owner invokes undefined behavior.

> To resume using the mutex you can initialize it again. In other
>words, you can invoke pthread_mutex_init() on this mutex. This will
>unlock the mutex for you.

Again per POSIX, reinitializing an initialized mutex invokes undefined
behavior, as does destroying a locked mutex.

Just as an example of the problems, what happens to all the threads that
were blocked waiting for the mutex that was just reinitialized out from
under them.

>This is sort of a dirty trick. If a thread had a mutex locked when
>it was killed it is very likely that the data (that the mutex
>protects) is in some inconsistent state. So, besides unlocking the
>mutex (by the means of pthread_mutex_init()) you also have to
>restore the data.

There are applications where that kind of roll-back is possible and even
necessary, just perhaps tricky to write. That's why there is interest in
so-called "robust" mutexes.

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