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

From: John David Anglin
Date: Sun Jun 01 2014 - 16:53:57 EST


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.

Dave
--
John David Anglin dave.anglin@xxxxxxxx



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