Re: [PATCH locking/Documentation 1/2] Add note of release-acquire store vulnerability

[Will Deacon]

On Thu, Sep 29, 2016 at 09:43:53AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 29, 2016 at 05:03:08PM +0100, Will Deacon wrote:
> > On Thu, Sep 29, 2016 at 05:58:17PM +0200, Peter Zijlstra wrote:
> > > On Thu, Sep 29, 2016 at 08:54:01AM -0700, Paul E. McKenney wrote:
> > > > If two processes are related by a RELEASE+ACQUIRE pair, ordering can be
> > > > broken if a third process overwrites the value written by the RELEASE
> > > > operation before the ACQUIRE operation has a chance of reading it.
> > > > This commit therefore updates the documentation to call this vulnerability
> > > > out explicitly.
> > > > 
> > > > Reported-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > > 
> > > > +     However, please note that a chain of RELEASE+ACQUIRE pairs may be
> > > > +     broken by a store by another thread that overwrites the RELEASE
> > > > +     operation's store before the ACQUIRE operation's read.
> > > 
> > > This is the powerpc lwsync quirk, right? Where the barrier disappears
> > > when it looses the store.
> > > 
> > > Or is there more to it? Its not entirely clear from the Changelog, which
> > > I feel should describe the reason for the behaviour.
> > 
> > If I've groked it correctly, it's for cases like:
> > 
> > 
> > PO:
> > Wx=1
> > WyRel=1
> > 
> > P1:
> > Wy=2
> > 
> > P2:
> > RyAcq=2
> > Rx=0
> > 
> > Final value of y is 2.
> > 
> > 
> > This is permitted on arm64. If you make P1's store a store-release, then
> > it's forbidden, but I suspect that's not generally true of the kernel
> > memory model.
> 
> That is the one!  And to Peter's point, powerpc does the same for the
> example as shown.  However, on powerpc, upgrading P1's store to release
> has no effect because there is no earlier access for the resulting
> lwsync to influence.  For whatever it might be worth, C11 won't guarantee
> ordering in that case, either.  Nor will the current Linux-kernel memory
> model.  (Yes, I did just try it to make sure.  Why do you ask?)
> 
> So you guys are fishing for an expanded commit log, for example, like
> the following?  ;-)
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> If two processes are related by a RELEASE+ACQUIRE pair, ordering can be
> broken if a third process overwrites the value written by the RELEASE
> operation before the ACQUIRE operation has a chance of reading it, for
> example:
> 
> 	P0(int *x, int *y)
> 	{
> 		WRITE_ONCE(*x, 1);
> 		smp_wmb();
> 		smp_store_release(y, 1);
> 	}
> 
> 	P1(int *y)
> 	{
> 		smp_store_release(y, 2);
> 	}
> 
> 	P2(int *x, int *y)
> 	{
> 		r1 = smp_load_acquire(y);
> 		r2 = READ_ONCE(*x);
> 	}
> 
> Both ARM and powerpc allow the "after the dust settles" outcome (r1=2 &&
> r2=0), as does the current version of the early prototype Linux-kernel
> memory model.

FWIW, ARM doesn't allow this and arm64 only allows it if P1 uses WRITE_ONCE
instead of store-release.

Will