Re: [RFC 3/3] mutex: When there is no owner, stop spinning after too many tries

[Jason Low <jason.low2@hp.com>]

On Thu, 2014-01-16 at 10:14 +0700, Linus Torvalds wrote:
> On Thu, Jan 16, 2014 at 9:45 AM, Jason Low <jason.low2@hp.com> wrote:
> >
> > Any comments on the below change which unlocks the mutex before taking
> > the lock->wait_lock to wake up a waiter? Thanks.
> 
> Hmm. Doesn't that mean that a new lock owner can come in *before*
> you've called debug_mutex_unlock and the lockdep stuff, and get the
> lock? And then debug_mutex_lock() will be called *before* the unlocker
> called debug_mutex_unlock(), which I'm sure confuses things.

If obtaining the wait_lock for debug_mutex_unlock is the issue, then
perhaps we can address that by taking care of
#ifdef CONFIG_DEBUG_MUTEXES. In the CONFIG_DEBUG_MUTEXES case, we can
take the wait_lock first, and in the regular case, take the wait_lock
after releasing the mutex.

#ifdef CONFIG_DEBUG_MUTEXES
	spin_lock_mutex(&lock->wait_lock, flags);
#endif

        mutex_release(&lock->dep_map, nested, _RET_IP_);
        debug_mutex_unlock(lock);
        if (__mutex_slowpath_needs_to_unlock())
                atomic_set(&lock->count, 1);

#ifndef CONFIG_DEBUG_MUTEXES
	spin_lock_mutex(&lock->wait_lock, flags);
#endif