Re: [PATCH] fix a race condition in cancelable mcs spinlocks

Paul E. McKenney
2014-06-01T21:47:08Z

On Sun, Jun 01, 2014 at 11:30:03PM +0200, Peter Zijlstra wrote:
> On Sun, Jun 01, 2014 at 04:46:26PM -0400, John David Anglin wrote:
> > On 1-Jun-14, at 3:20 PM, Peter Zijlstra 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.
> > >
> > >And this is really the first place in the kernel that breaks like this?
> > >I've been using xchg() and cmpxchg() without such consideration for
> > >quite a while.
> >
> > I believe Mikulas is correct. Even in a controlled situation where a
> > cmpxchg operation
> > is used to implement pthread_spin_lock() in userspace, we found recently
> > that the lock
> > must be released with a cmpxchg operation and not a simple write on SMP
> > systems.
> > There is a race in the cache operations or instruction ordering that's not
> > present with
> > the ldcw instruction.
>
> Oh, I'm not arguing that. He's quite right that its broken, but this
> form of atomic ops is also quite insane and unusual. Most sane machines
> don't have this problem.
>
> My main concern is how are we going to avoid breaking parisc (and I
> think sparc32, which is similarly retarded) in the future; we should
> invest in machinery to find and detect these things.

I cannot see an easy way to fix this by making ACCESS_ONCE() arch-dependent.
But could the compiler help out by recognizing ACCESS_ONCE() and generating
the needed code for it on sparc and pa-risc?

Thanx, Paul