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    SubjectRe: [PATCH v2 1/7] locking/rwsem: check for active writer/spinner before wakeup
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    On Fri, 2014-08-08 at 13:21 -0700, Davidlohr Bueso wrote:
    > On Fri, 2014-08-08 at 12:50 -0700, Jason Low wrote:
    > > > __visible __used noinline
    > > > @@ -730,6 +744,23 @@ __mutex_unlock_common_slowpath(struct mutex *lock, int nested)
    > > > if (__mutex_slowpath_needs_to_unlock())
    > > > atomic_set(&lock->count, 1);
    > > >
    > > > +/*
    > > > + * Skipping the mutex_has_owner() check when DEBUG, allows us to
    > > > + * avoid taking the wait_lock in order to do not call mutex_release()
    > > > + * and debug_mutex_unlock() when !DEBUG. This can otherwise result in
    > > > + * deadlocks when another task enters the lock's slowpath in mutex_lock().
    > > > + */
    > > > +#ifndef CONFIG_DEBUG_MUTEXES
    > > > + /*
    > > > + * Abort the wakeup operation if there is an another mutex owner, as the
    > > > + * lock was stolen. mutex_unlock() should have cleared the owner field
    > > > + * before calling this function. If that field is now set, another task
    > > > + * must have acquired the mutex.
    > > > + */
    > > > + if (mutex_has_owner(lock))
    > > > + return;
    > >
    > > Would we need the mutex lock count to eventually get set to a negative
    > > value if there are waiters? An optimistic spinner can get the lock and
    > > set lock->count to 0. Then the lock count might remain 0 since a waiter
    > > might not get waken up here to try-lock and set lock->count to -1 if it
    > > goes back to sleep in the lock path.
    >
    > This is a good point, but I think we are safe because we do not rely on
    > strict dependence between the mutex counter and the wait list. So to see
    > if there are waiters to wakeup, we do a !list_empty() check, but to
    > determine the lock state, we rely on the counter.

    Right, though if an optimistic spinner gets the lock, it would set
    lock->count to 0. After it is done with its critical region and calls
    mutex_unlock(), it would skip the slowpath and not wake up the next
    thread either, because it sees that the lock->count is 0. In that case,
    there might be a situation where the following mutex_unlock() call would
    skip waking up the waiter as there's no call to slowpath.




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