"Peter Dimov" <pdimov@pdimov.com> writes:
Anthony Williams:
"Peter Dimov" <pdimov@pdimov.com> writes:
I'm seeing a deadlock on a dual core Pentium D from libs/smart_ptr/test/sp_atomic_mt_test.cpp when USE_RWLOCK is defined (which causes it to use boost::shared_mutex). It works on uniprocessor.
How often, on what platform?
Every time, on a dual core Pentium D running XP64. The program is compiled as 32 bit with MSVC 7.1. It looks like a livelock rather than a deadlock, since the program keeps spinning its wheels at 100% but never completes. The writer thread is probably either blocked or starved. This happens on another (quad core) machine too for which I don't know the specs at the moment.
Here's the thing: your writer is running in a tight loop, and so are your readers, with each reacquiring the lock immediately after releasing it. shared_mutex is not designed for this scenario, since you have high contention. shared_mutex is designed for infrequent updates. Your test should complete eventually (it takes 2-5 minutes on my machine), but it is likely that the problem is indeed due to writer starvation. In the current implementation, a blocked writer will prevent further readers acquiring the lock, thus if you frequently try and obtain a writer lock this reduces the potential reader concurrency. However, in order to avoid reader starvation, the writer is not automatically granted the lock when the read lock is released: instead it has to compete for the privilege with the blocked readers. It is up to the OS how it chooses to schedule it. Amusingly, the way I've programmed the upgrade_lock means that your test runs quicker (22 seconds vs 2-5 minutes) if you do: boost::upgrade_lock<boost::shared_mutex> up_lock( rw ); boost::unique_lock<boost::shared_mutex> lock( up_lock.move() ); rather than boost::unique_lock<boost::shared_mutex> lock( rw ); Anthony -- Anthony Williams | Just Software Solutions Ltd Custom Software Development | http://www.justsoftwaresolutions.co.uk Registered in England, Company Number 5478976. Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL