Re: I/O memory barriers vs SMP memory barriers

[Paul E. McKenney]

On Fri, Mar 23, 2007 at 01:43:53PM +0000, David Howells wrote:
> 
> [Resend - this time with a comma in the addresses, not a dot]
> 
> Lennert Buytenhek <buytenh@xxxxxxxxxxxxxx> wrote:
> 
> > [ background: On ARM, SMP synchronisation does need barriers but device
> >   synchronisation does not.  The question is that given this, whether
> >   mb() and friends can be NOPs on ARM or not (i.e. whether mb() is
> >   supposed to sync against other CPUs or not, or whether only smp_mb()
> >   can be used for this.)  ]
> 
> Hmmmm...
> 
> I see your problem.  I think the right way to deal with this is to get rid of
> mb(), rmb(), wmb() and read_barrier_depends() and replace them with io_mb(),
> io_rmb(), ...

We will get combinatorial explosion if we aren't -extremely- careful:

1.	Orders only normal memory accesses, which is all that is required
	of smp_*().

2.	Orders both normal and device accesses -- mmiowb().

3.	Orders memory accesses and device accesses, but not necessarily
	the union of the two -- mb(), rmb(), wmb().

4.	Orders only device accesses, which is what seems to be looked
	for here.

						Thanx, Paul

> I think that there are only two places you should be using explicit memory
> barriers:
> 
>  (1) To control inter-CPU effects on an SMP system.
> 
>  (2) To control CPU vs device effects.
> 
> > On Thu, Mar 22, 2007 at 04:17:44PM +0000, Catalin Marinas wrote:
> > 
> > > Is the requirement for mb() to act correctly in the SMP case as well?
> > 
> > That's what the docs seem to suggest.  A couple of snippets from
> > memory-barriers.txt:
> > 
> > [1]  A write memory barrier gives a guarantee that all the STORE operations
> >      specified before the barrier will appear to happen before all the STORE
> >      operations specified after the barrier with respect to the other
> >      components of the system.
> > 
> > [2]  A read barrier is a data dependency barrier plus a guarantee that all the
> >      LOAD operations specified before the barrier will appear to happen before
> >      all the LOAD operations specified after the barrier with respect to the
> >      other components of the system.
> > 
> > [3]     TYPE            MANDATORY               SMP CONDITIONAL
> >         =============== ======================= ===========================
> >         GENERAL         mb()                    smp_mb()
> >         WRITE           wmb()                   smp_wmb()
> >         READ            rmb()                   smp_rmb()
> >         DATA DEPENDENCY read_barrier_depends()  smp_read_barrier_depends()
> > 
> > [4]  Mandatory barriers should not be used to control SMP effects,
> >      since mandatory barriers unnecessarily impose overhead on UP
> >      systems.
> > 
> > Note the wording of 'other components of the system' in [1] and [2] --
> > the way I read it, this includes devices as well as other CPUs.
> 
> Yes, but I suppose which "other components" may depend on the class of barrier
> used.
> 
> > [4] says that mandatory barriers (i.e. from [3]: mb(), wmb(), rmb(),
> > read_barrier_depends()) SHOULD not be used to control SMP effects, but
> > it does not say that they MUST not.
> 
> As it stands, mb() is a superset of smp_mb(), and rmb() of smp_rmb(), etc.,
> so, yes, currently, mb() implies smp_mb().  However, mb() shouldn't be used if
> smb_mb() is sufficient as that may impact performance on a UP system.
> 
> Really, mb() should only be used with respect to I/O.
> 
> > > The memory-barriers.txt doc says that smp_* must be used for the SMP
> > > case.
> > 
> > The exact wording is:
> > 
> > 	[!] Note that SMP memory barriers _must_ be used to control the
> > 	ordering of references to shared memory on SMP systems, though
> > 	the use of locking instead is sufficient.
> > 
> > This can IMHO be interpreted in two ways:
> > 1. If you want to control ordering of references to shared memory on
> >    SMP systems, you must use SMP memory barriers and not any other kind
> >    of memory barrier.
> 
> If the shared memory is purely an inter-CPU effect, yes.  If the shared memory
> is actually a device with side effects, then I/O safe memory barriers are
> required - mb() and co.  Note that there must _also_ be safety wrt to other
> CPUs in the system, as other CPUs may also try to access the device.
> 
> > 2. If you want to control ordering of references to shared memory on
> >    SMP systems, you must use memory barriers, and the SMP memory barrier
> >    is the most appropriate barrier type to use.
> 
> You may use locking instead to control inter-CPU effects.  Locks imply one-way
> permeable SMP-class memory barriers.
> 
> > I'm thinking that [2] is what was intended.  [1] doesn't seem consistent
> > with the rest of the document, but if [1] _is_ what is what was intended,
> > we're off the hook and mb() and friends can be NOPs on ARM.  (But it'd
> > probably still need a thorough audit... :-/ )
> 
> I think the best way to do an audit would be to make mb() and co. deprecated,
> pending obsolete, and to replace them with io_mb() and co.  That way people
> would have to eyeball any usages of mb() and co.
> 
> > > This means that if code uses mb() to control SMP sharing, it is broken.
> > 
> > I'm not so sure.
> 
> If it's _purely_ to control inter-CPU SMP sharing, then yes, it's broken.  It
> must use either a lock or an smp_*mb() barrier.
> 
> Of course, Linus may disagree...
> 
> David
> 
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