Re: [RFC PATCH 1/3] locking/mutex: Try to acquire mutex only if it is unlocked

[Long, Wai Man]

On 6/11/2014 5:48 PM, Jason Low wrote:
> On Wed, 2014-06-11 at 17:00 -0400, Long, Wai Man wrote:
> > On 6/9/2014 1:38 PM, Jason Low wrote:
> > > On Wed, 2014-06-04 at 13:58 -0700, Davidlohr Bueso wrote:
> > > > On Wed, 2014-06-04 at 13:57 -0700, Davidlohr Bueso wrote:
> > > > > In addition, how about the following helpers instead:
> > > > > - mutex_is_unlocked() : count > 0
> > > > > - mutex_has_waiters() : count < 0, or list_empty(->wait_list)
> > > >                                         ^ err, that's !list_empty()
> > > Between checking for (count < 0) or checking for !list_empty(wait_list)
> > > for waiters:
> > >
> > > Now that I think about it, I would expect a mutex_has_waiters() function
> > > to return !list_empty(wait_list) as that really tells whether or not
> > > there are waiters. For example, in highly contended cases, there can
> > > still be waiters on the mutex if count is 1.
> > >
> > > Likewise, in places where we currently use "MUTEX_SHOW_NO_WAITER", we
> > > need to check for (count < 0) to ensure lock->count is a negative value
> > > before the thread sleeps on the mutex.
> > >
> > > One option would be to still remove MUTEX_SHOW_NO_WAITER(), directly use
> > > atomic_read() in place of the macro, and just comment on why we have an
> > > extra atomic_read() that may "appear redundant". Another option could be
> > > to provide a function that checks for "potential waiters" on the mutex.
> > >
> > > Any thoughts?
> > For the first MUTEX_SHOW_NO_WAITER() call site, you can replace it with
> > a check for (count > 0).
> Yup, in my v2 patch, the first call site becomes !mutex_is_locked(lock)
> which is really a check for (count == 1).

Yes, your v2 patch looks fine to me.

> > The second call site within the for loop,
> > however, is a bit more tricky. It has to serve 2 purposes:
> >
> > 1. Opportunistically get the lock
> > 2. Set the count value to -1 to indicate someone is waiting on the lock,
> > that is why an xchg() operation has to be done even if its value is 0.
> >
> > I do agree that the naming isn't that good. Maybe it can be changed to
> > something like
> >
> > static inline int mutex_value_has_waiters(mutex *lock)    { return
> > lock->count < 0; }
> So I can imagine that a mutex_value_has_waiters() function might still
> not be a great name, since the mutex can have waiters in the case that
> the value lock->count >= 0.
>
> In the second call site, do you think we should just do a direct
> atomic_read(lock->count) >= 0 and comment that we only do the xchg if
> the count is non-negative to avoid unnecessary xchg? That what I did in
> my v2 patch.

I think that is a good idea to avoid any controversy in naming.

-Longman
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