Re: [Boost-users] [thread] Never enter critical section [NC]

From: frederic.mayot_at_[hidden]
Date: 2007-08-30 13:49:15

>>On Thu, Aug 30, 2007 at 12:08:51PM -0400, frederic.mayot_at_[hidden] 
wrote:
>> Hi,
>> I'm having a problem with mutexes.
>> 
>> In one thread T1 I have:
>> while(!end)
>> {
>>  /* TTT */
>>   lock lk(mutex);
>>   dosomethingquick();
>> }
>> 
>> 
>> I'm sure dosomethingquick returns but T2 can never acquires the mutex.
>> If I had a timer on line  /* TTT */ in T1 (sleeps for a millisecond), 
>> everything works fine.
>> 
>> Can anyone understand such a behavior? Am I doing something wrong here?
>> 
>
>A guess: T1 reacquires the mutex before the OS actually dispatches T2 to 
run.
>Unlocking the mutex just makes "ready" one of threads waiting for it, but 
I'm
>not sure that it's required that the thread be immediately dispatched it 
onto
>(a) CPU.  When you insert a volountary sleep in T1, the scheduler puts T1 
to
>sleep and chooses another ready thread (T2 in this case) to run.

Oh yes, I know that. The thing is I don't know what to do. The code I gave 
is, in my opinion, something really common. I'm just wondering how other 
people do in such a case... It seems completely absurd that I would have 
to add a sleep just to help a thread acquiring a mutex!!
The NPTL does not offer fair mutexes (hence, neither boost). Leaving my 
code as is, the only workaround I see would be to use an atomic variable 
v. T2 sets v to true at the beginning of  f() and sets it back to false at 
the end. T1 would sleep only if the variable is true. What an ugly patch!!

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