Hi Paul,
On Thu, Oct 08, 2015 at 03:17:16PM -0700, Paul E. McKenney wrote:
> On Thu, Oct 08, 2015 at 01:59:38PM +0100, Will Deacon wrote:
> > I thought Paul was talking about something like this case:
> >
> > CPU A CPU B CPU C
> > foo = 1
> > UNLOCK x
> > LOCK x
> > (RELEASE) bar = 1
> > ACQUIRE bar = 1
> > READ_ONCE foo = 0
>
> More like this:
>
> CPU A CPU B CPU C
> WRITE_ONCE(foo, 1);
> UNLOCK x
> LOCK x
> r1 = READ_ONCE(bar);
> WRITE_ONCE(bar, 1);
> smp_mb();
> r2 = READ_ONCE(foo);
>
> This can result in r1==0 && r2==0.
Thank you, that is extremely enlightening :)
> > I think we need a PPC litmus test illustrating the inter-thread, same
> > lock failure case when smp_mb__after_unlock_lock is not present so that
> > we can reason about this properly. Paul?
>
> Please see above. ;-)
>
> The corresponding litmus tests are below.
How do people feel about including these in memory-barriers.txt? I find
them considerably easier to read than our current kernel code + list of
possible orderings + wall of text, but there's a good chance that my
brain has been corrupted from staring at this stuff for too long.
The only snag is the ppc assembly code, but it's not *too* horrific ;)
> PPC lock-2thread-WR-barrier.litmus
> ""
> (*
> * Does 3.0 Linux-kernel Power lock-unlock provide local
> * barrier that orders prior stores against subsequent loads,
> * if the unlock and lock happen on different threads?
> * This version uses lwsync instead of isync.
> *)
> (* 23-July-2013: ppcmem says "Sometimes" *)
> {
> l=1;
> 0:r1=1; 0:r4=x; 0:r10=0; 0:r12=l;
> 1:r1=1; 1:r3=42; 1:r4=x; 1:r5=y; 1:r10=0; 1:r11=0; 1:r12=l;
> 2:r1=1; 2:r4=x; 2:r5=y;
> }
> P0 | P1 | P2;
> stw r1,0(r4) | lwarx r11,r10,r12 | stw r1,0(r5) ;
> lwsync | cmpwi r11,0 | lwsync ;
> stw r10,0(r12) | bne Fail1 | lwz r7,0(r4) ;
> | stwcx. r1,r10,r12 | ;
> | bne Fail1 | ;
> | isync | ;
> | lwz r3,0(r5) | ;
> | Fail1: | ;
>
>
> exists
> (1:r3=0 /\ 2:r7=0)
We could also include a link to the ppcmem/herd web frontends and your
lwn.net article. (ppcmem is already linked, but it's not obvious that
you can run litmus tests in your browser).
Will
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