Messages in this thread | | | Date | Thu, 26 Jan 2023 11:36:08 -0500 | From | Alan Stern <> | Subject | Re: [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po |
| |
On Thu, Jan 26, 2023 at 02:46:04PM +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 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 eliminating this possibility > from mb (but not strong-fence) and relying explicitly on mb|gp instead > of strong-fence when defining ppo. > > Signed-off-by: Jonas Oberhauser <jonas.oberhauser@huaweicloud.com> > ---
This changes the meaning of the fence relation, which is used in w-pre-bounded, w-post-bounded, ww-vis, wr-vis, and rw-xbstar. Have you checked that they won't be affected by the change?
> tools/memory-model/linux-kernel.cat | 9 +++++---- > 1 file changed, 5 insertions(+), 4 deletions(-) > > diff --git a/tools/memory-model/linux-kernel.cat b/tools/memory-model/linux-kernel.cat > index 6e531457bb73..815fdafacaef 100644 > --- a/tools/memory-model/linux-kernel.cat > +++ b/tools/memory-model/linux-kernel.cat > @@ -36,7 +36,9 @@ let wmb = [W] ; fencerel(Wmb) ; [W] > let mb = ([M] ; fencerel(Mb) ; [M]) | > ([M] ; fencerel(Before-atomic) ; [RMW] ; po? ; [M]) | > ([M] ; po? ; [RMW] ; fencerel(After-atomic) ; [M]) | > - ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) | > + ([M] ; po? ; [LKW] ; fencerel(After-spinlock) ; [M]) > +let gp = po ; [Sync-rcu | Sync-srcu] ; po? > +let strong-fence = mb | gp | > (* > * Note: The po-unlock-lock-po relation only passes the lock to the direct > * successor, perhaps giving the impression that the ordering of the > @@ -50,10 +52,9 @@ let mb = ([M] ; fencerel(Mb) ; [M]) | > *) > ([M] ; po-unlock-lock-po ; > [After-unlock-lock] ; po ; [M]) > -let gp = po ; [Sync-rcu | Sync-srcu] ; po? > -let strong-fence = mb | gp > > -let nonrw-fence = strong-fence | po-rel | acq-po > +
Extra blank line.
> +let nonrw-fence = mb | gp | po-rel | acq-po > let fence = nonrw-fence | wmb | rmb > let barrier = fencerel(Barrier | Rmb | Wmb | Mb | Sync-rcu | Sync-srcu | > Before-atomic | After-atomic | Acquire | Release | > -- > 2.17.1
Alan
| |