Re: [RFC,PATCH] mutex: mutex_is_owner() helper

From: Eric Dumazet
Date: Wed Nov 04 2009 - 12:19:51 EST

Ingo Molnar a écrit :

> To make sure this does not extend mutexes to be used a recursive
> mutexes, mind naming it more clearly, like debug_mutex_is_owned(), and
> adding a comment that says that this shouldnt be taken?
> Also, it's somewhat imprecise: on !SMP && !DEBUG_MUTEXES we might return
> a false '1'. Which happens to work for the rtnl usecase - but might not
> in other cases.

Sure, we can chose another name, but what do you mean by a false '1' ?

1 means mutex is locked and that we could not check ownership.
(best effort, ie same imprecise result than mutex_is_locked())

BTW, I was thinking of a mutex_yield() implementation, but could not
cook it without hard thinking, maybe you already have some nice implementation ?

We have some uses of "mutex_unlock();mutex_lock();" things that are
not working nicely because current thread immediately takes again mutex.

a true mutex_yield() would force current thread to go at the end of wait_list.

int mutex_yield(struct mutex *lock)
unsigned long flags;

// OK to test list without locking
if (list_empty(&lock->wait_list))
return 0;

spin_lock_mutex(&lock->wait_lock, flags);
if (!list_empty(&lock->wait_list)) {
atomic_xchg(&lock->count, 1);// free mutex
list_add_tail(&waiter.list, &lock->wait_list);//insert me at tail of wait_list
wake head of wait_list
__mutex_lock_common_condadd(mutex, TASK_UNINTERRUPTIBLE, DONT_ADD_TAIL, ...);
} else {
spin_unlock_mutex(&lock->wait_lock, flags);
return 1;

Or maybe we should try something less complex (slowpath anyway)

int mutex_yield(struct mutex *lock)
int ret = 0;

if (mutex_needbreak(lock) || should_resched()) {
ret = 1;
return ret;

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