Re: [Xen-devel] [PATCH] x86 spinlock: Fix memory corruption on completing completions

[Oleg Nesterov <oleg@xxxxxxxxxx>]

On 02/11, Jeremy Fitzhardinge wrote:
>
> On 02/11/2015 09:24 AM, Oleg Nesterov wrote:
> > I agree, and I have to admit I am not sure I fully understand why
> > unlock uses the locked add. Except we need a barrier to avoid the race
> > with the enter_slowpath() users, of course. Perhaps this is the only
> > reason?
>
> Right now it needs to be a locked operation to prevent read-reordering.
> x86 memory ordering rules state that all writes are seen in a globally
> consistent order, and are globally ordered wrt reads *on the same
> addresses*, but reads to different addresses can be reordered wrt to writes.
>
> So, if the unlocking add were not a locked operation:
>
>         __add(&lock->tickets.head, TICKET_LOCK_INC);          /* not locked */
>
>         if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
>             __ticket_unlock_slowpath(lock, prev);
>
> Then the read of lock->tickets.tail can be reordered before the unlock,
> which introduces a race:

Yes, yes, thanks, but this is what I meant. We need a barrier. Even if
"Every store is a release" as Linus mentioned.

> This *might* be OK, but I think it's on dubious ground:
>
>         __add(&lock->tickets.head, TICKET_LOCK_INC);          /* not locked */
>
>       /* read overlaps write, and so is ordered */
>         if (unlikely(lock->head_tail & (TICKET_SLOWPATH_FLAG << TICKET_SHIFT))
>             __ticket_unlock_slowpath(lock, prev);
>
> because I think Intel and AMD differed in interpretation about how
> overlapping but different-sized reads & writes are ordered (or it simply
> isn't architecturally defined).

can't comment, I simply so not know how the hardware works.

> If the slowpath flag is moved to head, then it would always have to be
> locked anyway, because it needs to be atomic against other CPU's RMW
> operations setting the flag.

Yes, this is true.

But again, if we want to avoid the read-after-unlock, we need to update
this lock and read SLOWPATH atomically, it seems that we can't avoid the
locked insn.

Oleg.


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