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Date: Sat, 6 Dec 2003 13:24:39 -0500 (EST)
From: Daniel Eischen <eischen@vigrid.com>
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To: Igor Sysoev <is@rambler-co.ru>
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Subject: Re: pthread_mutex_trylock() should never block
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On Sat, 6 Dec 2003, Igor Sysoev wrote:

> The current pthread_mutex_trylock() implementation in both libpthread (libkse)
> and libthr can block if mutex is locked by another thread.
> 
> libphtread calls THR_LOCK_ACQUIRE() and it can be blocked.  

That's an implementation detail.  Threads release the low-level
lock (THR_LOCK_ACQUIRE/THR_LOCK_RELEASE) after they take the mutex.
I don't believe we (libpthread) are conflicting with POSIX in
this respect.

> libthr's pthread_mutex_trylock() is too heavy - it calls 3 syscalls:
> sigprocmask()/umtx_lock()/sigprocmask() and can be blocked too.
> 
> 
> SUSv2 states:
> [ http://www.opengroup.org/onlinepubs/007908799/xsh/pthread_mutex_lock.html ]
> 
> --------
> The function pthread_mutex_trylock() is identical to pthread_mutex_lock()
> except that if the mutex object referenced by mutex is currently locked
> (by any thread, including the current thread), the call returns immediately.

The low-level locks are necessary (at least in libpthread implementation)
to safely test the mutex for ownership.

-- 
Dan Eischen