Re: WaitForSingleObject() will not deadlock
On Sun, 01 Jul 2007 23:57:05 -0700, Frank Cusack <fcusack@fcusack.com>
wrote:
Seems expensive then. A pthreads semaphore (typically used to guard a
critical section) has no memory visibility guarantees and as such can
be implemented with so-called atomic ops instead of memory barriers.
Cite?
The whole point of a "critical section" is to preserve data invariants. A
synchronization primitive that doesn't guarantee consistent view of the
data cannot be used to establish a critical section. You use a mutex to
guard a critical section.
According to Butenhof's book, pthreads does not provide semaphores; they
come from POSIX, which allows them to be used from signal handlers. On page
236, he says, "Semaphores are a general synchronization mechanism. We just
have no reason to use them that way." Instead, pthreads focuses on mutexes
and condition variables.
posix mutexes (good implementations, anyway) only require entry into
the kernel when they are contested. Uncontested mutexes are extremely
fast. Solaris has an "adaptive mutex" which only enters the kernel
after spinning for a bit first, which is consistent with the design
philosophy that you should only hold on to mutexes for very short
periods of time. (There are more rules but it's not important to my
point.)
The Windows CRITICAL_SECTION works the same way.
It almost doesn't make sense that CRITICAL_SECTIONs execute a membar
since Windows really only runs on x86, which is TSO. This is probably
just a nod towards more widespread use (ie, less skilled programmers)
who might otherwise get the semantics wrong.
Of course it doesn't execute a membar on architectures that don't require
it. A CRITICAL_SECTION is a mutex, albeit a process-local one. The
"semantics" of a mutex include memory visibility guarantees, so if there is
a "nod" to anything, it's in the direction of fulfilling a correctness
requirement. IOW, a "mutex" that does not perform a memory fence when
necessary would be a fundamentally broken, useless thing. A programmer who
uses a mutex but doesn't expect memory visibility doesn't understand
mutexes.
--
Doug Harrison
Visual C++ MVP