On Thu, Nov 28, 2013 at 06:27:12PM +0000, Paul E. McKenney wrote:
> On Thu, Nov 28, 2013 at 06:03:18PM +0000, Will Deacon wrote:
> > Hmm, without horrible hacks to keep track of whether we've done an
> > mb__before_spinlock() without a matching spinlock(), that's going to end up
> > with full-barrier + pointless half-barrier (similarly on the release path).
>
> We should be able to detect mb__before_spinlock() without a matching
> spinlock via static analysis, right?
>
> Or am I missing your point?
See below...
> > > Yes, we might need better names, but I believe that this approach does
> > > what you need.
> > >
> > > Thoughts?
> >
> > I still think we need to draw the distinction between ordering all accesses
> > against a lock and ordering an unlock against a lock. The latter is free for
> > arm64 (STLR => LDAR is ordered) but the former requires a DMB.
> >
> > Not sure I completely got your drift...
>
> Here is what I am suggesting:
>
> o mb__before_spinlock():
>
> o Must appear immediately before a lock acquisition.
> o Upgrades a lock acquisition to a full barrier.
> o Emits DMB on ARM64.
Ok, so that then means that:
mb__before_spinlock();
spin_lock();
on ARM64 expands to:
dmb ish
ldaxr ...
so there's a redundant half-barrier there. If we want to get rid of that, we
need mb__before_spinlock() to set a flag, then we could conditionalise
ldaxr/ldxr but it's really horrible and you have to deal with interrupts
etc. so in reality we just end up having extra barriers.
Or we have separate a spin_lock_mb() function.
> o mb_after_spinlock():
>
> o Must appear immediatly after a lock acquisition.
> o Upgrades an unlock+lock pair to a full barrier.
> o Emits a no-op on ARM64, as in "do { } while (0)".
> o Might need a separate flavor for queued locks on
> some platforms, but no sign of that yet.
Ok, so mb__after_spinlock() doesn't imply a full barrier but
mb__before_spinlock() does? I think people will get that wrong :)
Will
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