Re: spin_lock implicit/explicit memory barrier

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Wed, Aug 10, 2016 at 10:05:37AM +1000, Benjamin Herrenschmidt wrote:
> On Tue, 2016-08-09 at 20:52 +0200, Manfred Spraul wrote:
> > Hi Benjamin, Hi Michael,
> > 
> > regarding commit 51d7d5205d33 ("powerpc: Add smp_mb() to 
> > arch_spin_is_locked()"):
> > 
> > For the ipc/sem code, I would like to replace the spin_is_locked() with 
> > a smp_load_acquire(), see:
> > 
> > http://git.cmpxchg.org/cgit.cgi/linux-mmots.git/tree/ipc/sem.c#n367
> > 
> > http://www.ozlabs.org/~akpm/mmots/broken-out/ipc-semc-fix-complex_count-vs-simple-op-race.patch
> > 
> > To my understanding, I must now add a smp_mb(), otherwise it would be 
> > broken on PowerPC:
> > 
> > The approach that the memory barrier is added into spin_is_locked() 
> > doesn't work because the code doesn't use spin_is_locked().
> > 
> > Correct?
> 
> Right, otherwise you aren't properly ordered. The current powerpc locks provide
> good protection between what's inside vs. what's outside the lock but not vs.
> the lock *value* itself, so if, like you do in the sem code, use the lock
> value as something that is relevant in term of ordering, you probably need
> an explicit full barrier.
> 
> Adding Paul McKenney.

To amplify what Ben said...

Any CPU holding a given lock will see any previous accesses made under
the protection of that lock.

A CPU -not- holding the lock can see misordering.  As Ben noted, to
that non-lock-holding CPU it might appear that a write made under the
protection of that lock was made after the lock was released.  Similarly,
to that CPU it might appear that a load done under the protection of that
lock completed before the lock was acquired.  Finally, a CPU not holding
the lock might see a store by one CPU holding the lock as happening
after a load (from some other variable) by the next CPU holding that lock.

							Thanx, Paul