Re: [PATCH] padata: use smp_mb in padata_reorder to avoid orphaned padata jobs

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jul 16, 2019 at 02:53:09PM +0200, Andrea Parri wrote:

> C daniel-padata
> 
> { }
> 
> P0(atomic_t *reorder_objects, spinlock_t *pd_lock)
> {
> 	int r0;
> 
> 	spin_lock(pd_lock);
> 	spin_unlock(pd_lock);
> 	smp_mb();
> 	r0 = atomic_read(reorder_objects);
> }
> 
> P1(atomic_t *reorder_objects, spinlock_t *pd_lock, spinlock_t *reorder_lock)
> {
> 	int r1;
> 
> 	spin_lock(reorder_lock);
> 	atomic_inc(reorder_objects);
> 	spin_unlock(reorder_lock);
> 	//smp_mb();
> 	r1 = spin_trylock(pd_lock);
> }
> 
> exists (0:r0=0 /\ 1:r1=0)
> 
> It seems worth noticing that this test's "exists" clause is satisfiable
> according to the (current) memory consistency model.  (Informally, this
> can be explained by noticing that the RELEASE from the spin_unlock() in
> P1 does not provide any order between the atomic increment and the read
> part of the spin_trylock() operation.)  FWIW, uncommenting the smp_mb()
> in P1 would suffice to prevent this clause from being satisfiable; I am
> not sure, however, whether this approach is feasible or ideal... (sorry,
> I'm definitely not too familiar with this code... ;/)

Urgh, that one again.

Yes, you need the smp_mb(); although a whole bunch of architectures can
live without it. IIRC it is part of the eternal RCsc/RCpc debate.

Paul/RCU have their smp_mb__after_unlock_lock() that is about something
similar, although we've so far confinsed that to the RCU code, because
of how confusing that all is.