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

[Paul E. McKenney]

On Mon, Jun 02, 2014 at 05:19:39AM -0400, Mikulas Patocka wrote:
> 
> 
> On Sun, 1 Jun 2014, 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.
> 
> Grep the kernel for "\<xchg\>" and "\<cmpxchg\>" and replace them with 
> atomic types and atomic access functions.

Not so good for pointers, though.  Defeats type-checking, for one thing.
An example of this is use of xchg() for atomically enqueuing RCU callbacks
in kernel/rcu/tree_plugin.h.

I still like the idea of PA-RISC's compiler implementing ACCESS_ONCE()
as needed to make things work on that architecture.

							Thanx, Paul

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