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

On 06/17/2014 04:36 PM, Konrad Rzeszutek Wilk wrote:
On Sun, Jun 15, 2014 at 02:47:00PM +0200, Peter Zijlstra wrote:
Because the qspinlock needs to touch a second cacheline; add a pending
bit and allow a single in-word spinner before we punt to the second
Could you add this in the description please:

And by second cacheline we mean the local 'node'. That is the:
mcs_nodes[0] and mcs_nodes[idx]

Perhaps it might be better then to split this in the header file
as this is trying to not be a slowpath code - but rather - a
pre-slow-path-lets-try-if-we can do another cmpxchg in case
the unlocker has just unlocked itself.

So something like:

diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h
index e8a7ae8..29cc9c7 100644
--- a/include/asm-generic/qspinlock.h
+++ b/include/asm-generic/qspinlock.h
@@ -75,11 +75,21 @@ extern void queue_spin_lock_slowpath(struct qspinlock 
*lock, u32 val);
  static __always_inline void queue_spin_lock(struct qspinlock *lock)
-       u32 val;
+       u32 val, new;

        val = atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL);
        if (likely(val == 0))
+       /* 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;

No, it can leave like that. The unlock path will not clear the pending bit. We are trying to make the fastpath as simple as possible as it may be inlined. The complexity of the queue spinlock is in the slowpath.

Moreover, an cmpxchg followed immediately followed by another cmpxchg will just increase the level of memory contention when a lock is fairly contended. The chance of second cmpxchg() succeeding will be pretty low.


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