Re: [Xen-devel] [PATCH 03/11] qspinlock: Add pending bit

[Paolo Bonzini <pbonzini@xxxxxxxxxx>]

Il 17/06/2014 22:36, Konrad Rzeszutek Wilk ha scritto:
> +       /* One more attempt - but if we fail mark it as pending. */
> +       if (val == _Q_LOCKED_VAL) {
> +               new = Q_LOCKED_VAL |_Q_PENDING_VAL;
> +
> +               old = atomic_cmpxchg(&lock->val, val, new);
> +               if (old == _Q_LOCKED_VAL) /* YEEY! */
> +                       return;
> +               val = old;
> +       }
Note that Peter's code is in a for(;;) loop:


+       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;
+       }
+
+       /*
+        * we won the trylock
+        */
+       if (new == _Q_LOCKED_VAL)
+               return;

So what you'd have is basically:

        /*
         * One more attempt if no one is already in queue.  Perhaps
         * they have unlocked the spinlock already.
         */
        if (val == _Q_LOCKED_VAL && atomic_read(&lock->val) == 0) {
                old = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
                if (old == 0) /* YEEY! */
                        return;
                val = old;
        }

But I agree with Waiman that this is unlikely to trigger often enough. 
It does have to be handled in the slowpath for correctness, but the most 
likely path is (0,0,1)->(0,1,1).
Paolo

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