Re: [Xen-devel] [PATCH v11 06/16] qspinlock: prolong the stay in the pending bit path

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Jun 12, 2014 at 04:54:52PM -0400, Waiman Long wrote:
> If two tasks see the pending bit goes away and try to grab it with cmpxchg,
> there is no way we can avoid the contention. However, if some how the
> pending bit holder get the lock and another task set the pending bit before
> the current task, the spinlock value will become
> _Q_PENDING_VAL|_Q_LOCKED_VAL. The while loop will end and the code will
> blindly try to do a cmpxchg unless we check for this case before hand. This
> is what my code does by going back to the beginning of the for loop.

There is already a test for that; see the goto queue;

---

        /*
         * wait for in-progress pending->locked hand-overs
         *
         * 0,1,0 -> 0,0,1
         */
        if (val == _Q_PENDING_VAL) {
                while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
                        cpu_relax();
        }

        /*
         * trylock || pending
         *
         * 0,0,0 -> 0,0,1 ; trylock
         * 0,0,1 -> 0,1,1 ; pending
         */
        for (;;) {
                /*
                 * If we observe any contention; queue.
                 */
                if (val & ~_Q_LOCKED_MASK)
                        goto queue;

                new = _Q_LOCKED_VAL;
                if (val == new)
                        new |= _Q_PENDING_VAL;

                old = atomic_cmpxchg(&lock->val, val, new);
                if (old == val)
                        break;

                val = old;
        }




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