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Mutex and posix
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Date: 2005-01-12 (17:29)
From: Luca Pascali <luca@b...>
Subject: Re: [Caml-list] Mutex and posix
Xavier Leroy wrote:

>The latter is a recent addition to the POSIX threads API -- it's not
>in the original POSIX threads spec (POSIX 1003.1c-1995).  I wouldn't
>rely on this function being available in all POSIX threads
I didn't know this.

>>Polling continously is different. If I have two threads that are running 
>>with scantimes one multiple of the other, it is possible that one of the 
>>two threads (the slower one) fails always or almost always the try_lock 
>It's hard to give useful suggestions without knowing more about your
>application, but it could be the case that you're using mutexes to do
>things they are not really designed for, i.e. plain mutual exclusion,
>for which neither trylock nor timedlock are needed.
>Maybe your application needs a more complex but better suited
>synchronization mechanism, which can generally be built on top of
>mutexes and conditions, or (at a higher semantic level) Concurrent
>ML-style events.
>- Xavier Leroy
I haven't an application.
I was just thinking about how I could port some programs organizations 
into Ocaml.

The case that rised my question was:
If I have a shared resource (let's say a pipe or a queue, or a generic 
file descriptor) for performing, for example, asyncronous communication 
between threads, it's quite dangerous in terms of deadlocks to perform 
myself a polling or locking one thread indefinetly until the resource is 
Polling with try_lock can bring to a deadlock or to a long freezing if 
one thread locks the mutex periodically, let's say, every 100 ms for 10 
ms and the other one looks for the same mutex periodically about every 
1200 ms. The probabilty to fall into the locking period is incredibly high.

If I'm not wrong, the timed mutex is handled by the operating system, so 
when the mutex is released the OS directly assigns it to the first 
waiting requesting thread, at least if the mutex cannot be locked during 
the specified time, function returns.

Anyway, knowing that this function is not actually available (I hope it 
will be in the future), it's obvious that I will look for another 
solution, maybe changing the syncronization method.

Thanks for the answer


Luca Pascali

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