Re: Question on smp_mb__before_spinlock

[Peter Zijlstra]

On Mon, Sep 05, 2016 at 03:37:14AM -0700, Paul E. McKenney wrote:
> On Mon, Sep 05, 2016 at 11:37:53AM +0200, Peter Zijlstra wrote:
> > Hi all,
> > 
> > So recently I've had two separate issues that touched upon
> > smp_mb__before_spinlock().
> > 
> > 
> > Since its inception, our understanding of ACQUIRE, esp. as applied to
> > spinlocks, has changed somewhat. Also, I wonder if, with a simple
> > change, we cannot make it provide more.
> > 
> > The problem with the comment is that the STORE done by spin_lock isn't
> > itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
> > it and cross with any prior STORE, rendering the default WMB
> > insufficient (pointed out by Alan).
> > 
> > Now, this is only really a problem on PowerPC and ARM64, the former of
> > which already defined smp_mb__before_spinlock() as a smp_mb(), the
> > latter does not, Will?
> > 
> > The second issue I wondered about is spinlock transitivity. All except
> > powerpc have RCsc locks, and since Power already does a full mb, would
> > it not make sense to put it _after_ the spin_lock(), which would provide
> > the same guarantee, but also upgrades the section to RCsc.
> > 
> > That would make all schedule() calls fully transitive against one
> > another.
> > 
> > 
> > That is, would something like the below make sense?
> 
> Looks to me like you have reinvented smp_mb__after_unlock_lock()...

Will said the same, but that one doesn't in fact do the first bit, as
ARM64 also needs a full barrier for that, while it doesn't need that to
upgrade to RCsc.