Re: [Threads-devel] boost::condition and Lock concept

From: Anthony Williams (anthony_at_[hidden])
Date: 2007-12-06 11:14:40

Quoting Frank Mori Hess <fmhess_at_[hidden]>:

> On Thursday 06 December 2007 03:02, Anthony Williams wrote:
>> Quoting Frank Mori Hess <frank.hess_at_[hidden]>:
>> > Is it possible to make condition_variable_any work with recursive
>> > mutexes?  It
>> > seems like an additional interface on the lock would be required,
>> > since a recursive mutex may still be locked by the current thread even
>> > after calling unlock().
>>
>> I think that would be dangerous. If you don't know how many times
>> you've locked the recursive mutex when you call wait() (in order to
>> unlock all but one prior to the call), then you might be unlocking the
>> mutex when a layer above you is expecting it to still be locked and
>> has incomplete changes that are protected by the lock.
>
> Okay, I suggest a sentence be added to the condition docs to explicitly
> state it is unsafe (will probably deadlock) to wait on a recursively
> locked mutex.

That's a good idea.

> I wonder if a sanity check could be added to condition::wait() to test for
> this case.  It would require a generalization of owns_lock(), which
> instead of returning a bool, returns an integer indicating how many times
> the current thread has recursively locked the mutex.  Then wait() could
> check it returns exactly 1.

That's something worth bearing in mind. There's nothing to stop  
condition::wait being specialized for a lock on a recursive mutex to  
do that.

Anthony
-- 
Anthony Williams
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