Re: [PATCH 4/9] firewire: don't use PREPARE_DELAYED_WORK

[Peter Hurley]

On 02/21/2014 11:57 AM, Tejun Heo wrote:
> Yo,
>
> On Fri, Feb 21, 2014 at 11:53:46AM -0500, Peter Hurley wrote:
> > Ok, I can do that. But AFAIK it'll have to be an smp_rmb(); there is
> > no mb__after unlock.
>
> We do have smp_mb__after_unlock_lock().
>
> > [ After thinking about it some, I don't think preventing speculative
> >    writes before clearing PENDING if useful or necessary, so that's
> >    why I'm suggesting only the rmb. ]
>
> But smp_mb__after_unlock_lock() would be cheaper on most popular
> archs, I think.

smp_mb__after_unlock_lock() is only for ordering memory operations
between two spin-locked sections on either the same lock or by
the same task/cpu. Like:

   i = 1
   spin_unlock(lock1)
   spin_lock(lock2)
   smp_mb__after_unlock_lock()
   j = 1

This guarantees that the store to j happens after the store to i.
Without it, a cpu can

   spin_lock(lock2)
   j = 1
   i = 1
   spin_unlock(lock1)

Regards,
Peter Hurley
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