Re: [PATCH v3] tools/memory-model: Make ppo a subrelation of po

[Joel Fernandes]

On Fri, Feb 24, 2023 at 02:52:51PM +0100, Jonas Oberhauser wrote:
> As stated in the documentation and implied by its name, the ppo
> (preserved program order) relation is intended to link po-earlier
> to po-later instructions under certain conditions.  However, a
> corner case currently allows instructions to be linked by ppo that
> are not executed by the same thread, i.e., instructions are being
> linked that have no po relation.
> 
> This happens due to the mb/strong-fence/fence relations, which (as
> one case) provide order when locks are passed between threads
> followed by an smp_mb__after_unlock_lock() fence.  This is
> illustrated in the following litmus test (as can be seen when using
> herd7 with `doshow ppo`):
> 
> P0(int *x, int *y)
> {
>     spin_lock(x);
>     spin_unlock(x);
> }
> 
> P1(int *x, int *y)
> {
>     spin_lock(x);
>     smp_mb__after_unlock_lock();
>     *y = 1;
> }
> 
> The ppo relation will link P0's spin_lock(x) and P1's *y=1, because
> P0 passes a lock to P1 which then uses this fence.
> 
> The patch makes ppo a subrelation of po by letting fence contribute
> to ppo only in case the fence links events of the same thread.
> 
> Signed-off-by: Jonas Oberhauser <jonas.oberhauser@xxxxxxxxxxxxxxx>
> ---
>  tools/memory-model/linux-kernel.cat | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
> index cfc1b8fd46da..adf3c4f41229 100644
> --- a/tools/memory-model/linux-kernel.cat
> +++ b/tools/memory-model/linux-kernel.cat
> @@ -82,7 +82,7 @@ let rwdep = (dep | ctrl) ; [W]
>  let overwrite = co | fr
>  let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
>  let to-r = (addr ; [R]) | (dep ; [Marked] ; rfi)
> -let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
> +let ppo = to-r | to-w | (fence & int) | (po-unlock-lock-po & int)

Alternatively can be the following appended diff? Requires only single 'int'
in ->ppo then and prevents future similar issues caused by sub relations.
Also makes clear that ->ppo can only be CPU-internal.

Or would that not work for some reason? For the test you shared at least, the
graphs are the same.

Either way:

Tested-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
Reviewed-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>

---8<-----------------------

diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat
index 07f884f9b2bf..63052d1628e9 100644
--- a/tools/memory-model/linux-kernel.cat
+++ b/tools/memory-model/linux-kernel.cat
@@ -70,7 +70,7 @@ let rwdep = (dep | ctrl) ; [W]
 let overwrite = co | fr
 let to-w = rwdep | (overwrite & int) | (addr ; [Plain] ; wmb)
 let to-r = addr | (dep ; [Marked] ; rfi)
-let ppo = to-r | to-w | fence | (po-unlock-lock-po & int)
+let ppo = (to-r | to-w | fence | po-unlock-lock-po) & int
 
 (* Propagation: Ordering from release operations and strong fences. *)
 let A-cumul(r) = (rfe ; [Marked])? ; r