Messages in this thread Patch in this message | | | From | Jonas Oberhauser <> | Subject | [PATCH v2 2/2] tools/memory-model: Make ppo a subrelation of po | Date | Thu, 26 Jan 2023 14:46:04 +0100 |
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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> --- 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 + +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
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