Re: [boost] read_write_mutex fundamentally broken in 1.33?

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2005-09-11 15:30:19

Howard Hinnant <hinnant_at_[hidden]> writes:

> On Sep 10, 2005, at 5:53 PM, Anthony Williams wrote:
>
>> Attached is my latest offering, for win32. It still doesn't support 
>> timed
>> locks or anything beyond basic read/write/upgradeable locks. However, 
>> it is
>> faster than my earlier offering, since it uses interlocked operations 
>> to avoid
>> OS locks where possible.
>>
>> With each attempt to lock, then the locking thread will wait until it 
>> can
>> enter the "pending active" state. Only one thread is permitted in this 
>> state
>> at a time. Once in this state, the locking thread will wait until the 
>> mutex
>> state is correct for the lock to be acquired. The mutex state is 
>> managed as a
>> state flag, and a reader count.
>
> This looks like a significant improvement over your previous offering.  
> But I'm still concerned.  But admittedly my analysis is incomplete, so 
> please forgive me if I've got it wrong:
>
> With both the previous release and this one it looks like those threads 
> waiting on upgradable or exclusive access are essentially polling if 
> another thread already enjoys such access.  What I'm not seeing is a 
> signal or condition variable upon which a thread sleeps after it enters 
> the first gate and tries to acquire upgradable or exclusive access but 
> discovers such access is not immediately available.  I fear the polling 
> behavior will defeat the optimizations that read/write/upgradable exist 
> for in the first place.  But I have not measured your code, so I could 
> well be mistaken.

You're half right! If the threads block on trying to enter a new state, then
they wait on the mutex_state_sem. This semaphore is triggered every time a
lock is released, and again if a waiting thread is awakened but cannot satisfy
it's condition.

This can lead to a thread polling, if it is the only waiting thread, because
once it has been awakened, it will keep releasing the semaphore and
reacquiring it until it's condition can be satisfied.

Thanks for your comments, I guess there's work to be done!

Anthony
-- 
Anthony Williams
Software Developer
Just Software Solutions Ltd
http://www.justsoftwaresolutions.co.uk