Christopher Friesen wrote:
Howard Chu wrote:
Robert Hancock wrote:
> This says nothing about requiring a reschedule. The "scheduling
policy"
> can well decide that the thread which just released the mutex can
> re-acquire it.
No, because the thread that just released the mutex is obviously not
one of the threads blocked on the mutex. When a mutex is unlocked,
one of the *waiting* threads at the time of the unlock must acquire
it, and the scheduling policy can determine that. But the thread the
released the mutex is not one of the waiting threads, and is not
eligible for consideration.
Is it *required* that the new owner of the mutex is determined at the
time of mutex release?
If the kernel doesn't actually determine the new owner of the mutex
until the currently running thread swaps out, it would be possible for
the currently running thread to re-aquire the mutex.
The SUSv3 text seems pretty clear. It says "WHEN pthread_mutex_unlock()
is called, ... the scheduling policy SHALL decide ..." It doesn't say
MAY, and it doesn't say "some undefined time after the call." There is
nothing optional or implementation-defined here. The only thing that is
not explicitly stated is what happens when there are no waiting threads;
in that case obviously the running thread can continue running.
re: forcing the mutex to ping-pong between different threads - if that
is inefficient, then the thread scheduler needs to be tuned differently.
Threads and thread context switches are supposed to be cheap, otherwise
you might as well just program with fork() instead. (And of course, back
when Unix was first developed, *processes* were lightweight, compared to
other extant OSs.)
--
-- Howard Chu
Chief Architect, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc
OpenLDAP Core Team http://www.openldap.org/project/
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