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

[Peter Hurley]

On 02/22/2014 01:43 PM, James Bottomley wrote:
> On Fri, 2014-02-21 at 18:01 -0500, Peter Hurley wrote:
> > 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)
>
> No the CPU cannot.  If the CPU were allowed to reorder locking
> sequences, we'd get speculation induced ABBA deadlocks.  The rules are
> quite simple: loads and stores cannot speculate out of critical
> sections.

If you look carefully, you'll notice that the stores have not been
moved from their respective critical sections; simply that the two
critical sections overlap because they use different locks.

Regards,
Peter Hurley

PS - Your reply address is unroutable.

> All architectures have barriers in place to prevent this ...
> I know from personal experience because the barriers on PARISC were
> originally too weak and we did get some speculation out of the critical
> sections, which was very nasty to debug.
>
> Stuff may speculate into critical sections from non-critical but never
> out of them and critical section boundaries may not reorder to cause an
> overlap.

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