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    SubjectRe: [RFC][PATCH -v3 10/10] locking/mutex: Implement alternative HANDOFF
    On Fri, Sep 09, 2016 at 06:32:04PM -0400, Waiman Long wrote:

    > I have another way of catching the uncleared handoff flag. See the following
    > code to see if you think that will work.
    >
    > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
    > index 9492494..362ff83 100644
    > --- a/kernel/locking/mutex.c
    > +++ b/kernel/locking/mutex.c
    > @@ -85,7 +85,13 @@ static inline bool __mutex_trylock(struct mutex *lock,
    > const
    >
    > owner = atomic_long_read(&lock->owner);
    > for (;;) { /* must loop, can race against a flag */
    > - unsigned long old;
    > + unsigned long old, flags = __owner_flags(owner);
    > +
    > + /*
    > + * We don't need to keep the HANDOFF flag for the waiter.
    > + */
    > + if (handoff)
    > + flags &= ~MUTEX_FLAG_HANDOFF;
    >
    > if (__owner_task(owner)) {
    > if (handoff && unlikely(__owner_task(owner) == current))

    I placed this condition below the __owner_task() branch.

    > @@ -688,7 +694,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned
    > * state back to RUNNING and fall through the next schedule(),
    > * or we must see its unlock and acquire.
    > */
    > - if (__mutex_trylock(lock, true))
    > + if (__mutex_trylock(lock, first))
    > break;
    >
    > spin_lock_mutex(&lock->wait_lock, flags);

    Hmm, yes. I think that works.

    We (the first waiter) set the flag, we clear it on try-lock, or unlock
    clears it when it hands the thing off.

    Much simpler. Thanks!

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