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

[Peter Hurley]

On 02/21/2014 06:18 PM, Tejun Heo wrote:
> On Fri, Feb 21, 2014 at 06:01:29PM -0500, Peter Hurley wrote:
> > 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)
> ;
> Hmmm?  I'm pretty sure that's a full barrier.  Local processor is
> always in order (w.r.t. the compiler).

It's a long story but the short version is that
Documentation/memory-barriers.txt recently was overhauled to reflect
what cpus actually do and what the different archs actually
deliver.

Turns out that unlock + lock is not guaranteed by all archs to be
a full barrier. Thus the smb_mb__after_unlock_lock().

This is now all spelled out in memory-barriers.txt under the
sub-heading "IMPLICIT KERNEL MEMORY BARRIERS".

Regards,
Peter Hurley

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