Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks

From: Jason Low
Date: Mon Jun 02 2014 - 12:50:24 EST

On Mon, 2014-06-02 at 12:00 -0400, Mikulas Patocka wrote:
> If you write to some variable with ACCESS_ONCE and use cmpxchg or xchg at
> the same time, you break it. ACCESS_ONCE doesn't take the hashed spinlock,
> so, in this case, cmpxchg or xchg isn't really atomic at all.

So if the problem is using ACCESS_ONCE writes with cmpxchg and xchg at
the same time, would the below change address this problem?

diff --git a/kernel/locking/mcs_spinlock.c b/kernel/locking/mcs_spinlock.c
index 838dc9e..8396721 100644
--- a/kernel/locking/mcs_spinlock.c
+++ b/kernel/locking/mcs_spinlock.c
@@ -71,7 +71,7 @@ bool osq_lock(struct optimistic_spin_queue **lock)
if (likely(prev == NULL))
return true;

- ACCESS_ONCE(prev->next) = node;
+ xchg(&prev->next, node);

* Normally @prev is untouchable after the above store; because at that
@@ -144,7 +144,7 @@ unqueue:

ACCESS_ONCE(next->prev) = prev;
- ACCESS_ONCE(prev->next) = next;
+ xchg(&prev->next, next);

return false;

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