Messages in this thread | | | Date | Tue, 13 Oct 2015 16:50:25 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt |
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On 10/13/2015 03:56 PM, Peter Zijlstra wrote: > On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote: > >> +gotlock: >> /* >> + * We now have the lock. We need to either clear the tail code or >> + * notify the next one in queue as the new queue head. >> */ >> + old = atomic_read(&lock->val); >> + while ((old& _Q_TAIL_MASK) == tail) { >> + int val; >> + int new = old& ~_Q_TAIL_MASK; >> + >> + /* >> + * We are the only one in the queue, so clear the tail code >> + * and return. >> + */ >> + val = atomic_cmpxchg(&lock->val, old, new); >> + if (old == val) >> + goto done; >> + old = val; >> + } >> + > This i need to think about a wee bit; its almost the same... > > > So the below is exactly duplicated from the normal slowpath, so why > don't you keep that there? > > It would get you something like: > > if (pv_wait_head_or_steal(..)) > goto stolen; > > > stolen: >> + /* >> + * contended path; wait for next, release. >> + */ >> + while (!(next = READ_ONCE(node->next))) >> + cpu_relax(); >> + >> + arch_mcs_spin_unlock_contended(&next->locked); >> + pv_kick_node(lock, next); > release: > ...
Yes, it is largely the same. I thought that you don't like too much change in the logic flow of the generic qspinlock code. I will make the change in the next revision.
Cheers, Longman
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