| Date | Wed, 18 Jun 2014 13:29:48 +0200 | From | Paolo Bonzini <> | Subject | Re: [PATCH 03/11] qspinlock: Add pending bit |
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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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