From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2] introduce atomic_pointer to fix a race condition in cancelable mcs spinlocks Date: Mon, 2 Jun 2014 10:03:27 -0700 Message-ID: <20140602170327.GV22231@linux.vnet.ibm.com> References: <1401727810.7440.34.camel@j-VirtualBox> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Mikulas Patocka , Linus Torvalds , Peter Zijlstra , jejb@parisc-linux.org, deller@gmx.de, John David Anglin , linux-parisc@vger.kernel.org, linux-kernel@vger.kernel.org, chegu_vinod@hp.com, Waiman.Long@hp.com, tglx@linutronix.de, riel@redhat.com, akpm@linux-foundation.org, davidlohr@hp.com, hpa@zytor.com, andi@firstfloor.org, aswin@hp.com, scott.norton@hp.com To: Jason Low Return-path: In-Reply-To: <1401727810.7440.34.camel@j-VirtualBox> List-ID: List-Id: linux-parisc.vger.kernel.org On Mon, Jun 02, 2014 at 09:50:10AM -0700, 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? And one could use cmpxchg() or atomic_add_return(..., 0) to read a value out. Probably at the cost of some performance impact, though. Thanx, Paul > ----- > 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; > } > >