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From: Kai Brüning (kai_at_[hidden])
Date: 2007-07-23 10:26:17


I have been using a two-layer wrapper for boost mutex recently. One layer to create levelled mutexes as described by Anthony, and another one to track the locked/unlocked state together with the owning thread for debug purposes.

I am contemplating making this a boost library and would be very happy to work on something together with you. What do you think about a general mutex wrapping framework allowing multiple wrappers plus a collection of useful wrappers such as those described here?

>From working with shared data for a while, I got the impression that any support for handling the deadlock problem would be very helpful indeed. To generalize Anthony's remark, probably no solution will be usable for every situation, therefore a flexible collection of concepts should be appropriate.


>Frank Mori Hess <fmhess_at_[hidden]> writes:
>> I'm toying with the idea of making a boost mutex wrapper class that tracks
>> and reports lock-ordering problems at runtime. The tracking might be
>> turned on/off with a preprocessor define. I'm curious if the experts
>> think this is worthwhile, or something better done at another level (like
>> using helgrind for instance)? It would use BGL to build up a global
>> directed graph that has the lock ordering information. As long as the
>> graph remains acyclic, there should be no lock ordering problems. Each
>> thread would have to maintain its own list of the mutexes it currently has
>> locked.
>That's an interesting idea --- by keeping track of the order in which locks
>are taken, you can identify potential sources of deadlock before they
>occur. In order to verify that they are indeed sources of deadlock, the
>results will need to be checked manually afterwards, to verify that
>potentially-deadlocking regions of code are running concurrently --- it is
>possible that one thread is finished before the other is started. However, it
>would be a potentially useful tool.
>I have code for a mutex that checks for deadlock at runtime --- every thread
>that tries to lock a checked mutex registers that it is waiting for that
>mutex, and then when the lock is acquired it flags the mutex to identify the
>owning thread. When a thread registers that it is waiting on a mutex, the
>chain is followed --- which thread currently owns the mutex? Is it waiting on
>another mutex? and so forth, until we either reach a thread that is not
>waiting (we're fine), or a thread that is waiting on a mutex owned by the
>original thread (deadlock).
>I also have an incomplete prototype of a "levelled" mutex --- each mutex is
>assigned a level, and threads may only lock a mutex with a higher level than
>any currently locked. This enforces an ordering on mutex locks, which will
>therefore prevent deadlock. It is not something that is usable in every
>situation, though.
>Anthony Williams
>Just Software Solutions Ltd -
>Registered in England, Company Number 5478976.
>Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL
>threads-devel mailing list

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