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 13:05 +0100, Peter Zijlstra wrote:
> On Wed, Jan 15, 2014 at 10:46:17PM -0800, Jason Low wrote:
> > 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.
> 
> I think we're already good for DEBUG_MUTEXES, because DEBUG_MUTEXES has
> to work for archs that have !__mutex_slowpath_needs_to_unlock() and also
> the DEBUG_MUTEXES code is entirely serialized on ->wait_lock.

Yeah, in the !__mutex_slowpath_needs_to_unlock() case, we release the
mutex before calling __mutex_unlock_common_slowpath().

Jason