Messages in this thread | | | Date | Mon, 02 Jun 2014 13:25:29 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks |
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On 06/02/2014 12:50 PM, Jason Low wrote: > 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; > } > >
Doing an xchg is a very expensive operation compared with ACCESS_ONCE. I will not suggest doing that to make it right for PA-RISC at the expense of performance in other architectures.
-Longman
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