[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH 10/11] qspinlock: Paravirt support

On 06/18/2014 08:03 AM, Paolo Bonzini wrote:
Il 17/06/2014 00:08, Waiman Long ha scritto:
+void __pv_queue_unlock(struct qspinlock *lock)
+    int val = atomic_read(&lock->val);
+    native_queue_unlock(lock);
+    if (val & _Q_LOCKED_SLOW)
+        ___pv_kick_head(lock);

Again a race can happen here between the reading and writing of the lock
value. I can't think of a good way to do that without using cmpxchg.

Could you just use xchg on the locked byte?


The slowpath flag is just an indication that the queue head cpu might have been suspended. It may not be due to spurious wakeup. Releasing the lock unconditionally may cause the queue to be changed while it is being inspected. It really depending on how the cpu kicking is being handled. My patch delays the unlocking until all the inspections had been done to make sure that we don't waste time doing a cpu kick that is not needed.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.