Re: sched: memory corruption on completing completions

[Davidlohr Bueso <dave@stgolabs.net>]

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