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

[Mikulas Patocka]

On Mon, 2 Jun 2014, Mikulas Patocka wrote:

> 
> 
> On Sun, 1 Jun 2014, 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.
> > 
> > Dave
> > --
> > John David Anglin	dave.anglin@xxxxxxxx
> 
> That is strange.
> 
> Spinlock with cmpxchg on lock and a single write on unlock should work,
> assuming that cmpxchg doesn't write to the target address when it detects
> mismatch (the cmpxchg in the kernel syscall page doesn't do it, it
> nullifies the write instruction on mismatch).
> 
> Do you have some code that reproduces this misbehavior?
> 
> We really need to find out why does it behave this way:
> - is PA-RISC really out of order? (we used to believe that it is in-order
>   and we have empty barrier instructions in the kernel). Does adding the
>   "SYNC" instruction before the write in pthread_spin_unlock fix it?
> - does the processor performs nullified writes unconditionally? Does
>   moving the write in the cmpxchg implementation from the nullified slot
>   to is own branch fix it?
> - does adding a dummy "ldcw" instruction to an unrelated address fix it?
>   Is it that "ldcw" has some magic barrier properties?

- and there is "stw,o" instruction that does ordered store according to 
the specification, so we should test it too...

> I think we need to perform these tests and maybe some more to find out
> what really happened there...
> 
> BTW. in Debian 5 libc 2.7, pthread_spin_lock uses ldcw and 
> pthread_spin_unlock uses a single write (just like the kernel spinlock 
> implementation). In Debian-ports libc 2.18, both pthread_spin_lock and 
> pthread_spin_unlock call the kernel syscall page. What was the reason for 
> switching to a less efficient implementation?
> 
> Mikulas
> 
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