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SubjectRe: sched: memory corruption on completing completions
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On Wed, 2015-02-04 at 16:16 -0800, Linus Torvalds wrote:
> And looking at the arch version, I think the paravirtualized code is crap.
>
> It does:
>
> prev = *lock;
> add_smp(&lock->tickets.head, TICKET_LOCK_INC);
>
> /* add_smp() is a full mb() */
>
> if (unlikely(lock->tickets.tail & TICKET_SLOWPATH_FLAG))
> __ticket_unlock_slowpath(lock, prev);
>
>
> which is *exactly* the kind of things you cannot do with spinlocks,
> because after you've done the "add_smp()" and released the spinlock
> for the fast-path, you can't access the spinlock any more. Exactly
> because a fast-path lock migth come in, and release the whole data
> structure.
>
> As usual, the paravirt code is a horribly buggy heap of crud. Film at 11.

Per http://lwn.net/Articles/495597/ which clearly describes the intent
of the slowpath unlocking. Cc'ing Raghavendra.

Thanks,
Davidlohr



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