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[PATCH] xen/arm64/atomic: Clear exclusive monitor on cmpxchg failure
- To: xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Ryoji Okamoto <okamoto@xxxxxxxxxxxxx>
- Date: Wed, 9 Sep 2026 14:52:37 +0900
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- Cc: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>, Ryoji Okamoto <okamoto@xxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>
- Delivery-date: Wed, 09 Sep 2026 05:53:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
When the value comparison fails in atomic_cmpxchg, the code branches
out without executing stxr, leaving the exclusive monitor in the
exclusive state set by ldxr.
Add `clrex` to the failure path to explicitly clear the exclusive
monitor.
Fixes: d2654a556835 ("xen: arm64: atomics")
Signed-off-by: Ryoji Okamoto <okamoto@xxxxxxxxxxxxx>
---
xen/arch/arm/include/asm/arm64/atomic.h | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)
diff --git a/xen/arch/arm/include/asm/arm64/atomic.h
b/xen/arch/arm/include/asm/arm64/atomic.h
index 4460165295..2c74ede740 100644
--- a/xen/arch/arm/include/asm/arm64/atomic.h
+++ b/xen/arch/arm/include/asm/arm64/atomic.h
@@ -118,7 +118,9 @@ static inline int atomic_cmpxchg(atomic_t *v, int old, int
new)
" b.ne 2f\n"
" stxr %w0, %w4, %2\n"
" cbnz %w0, 1b\n"
-"2:"
+" b 3f\n"
+"2: clrex\n"
+"3:"
: "=&r" (tmp), "=&r" (oldval), "+Q" (v->counter)
: "Ir" (old), "r" (new)
: "cc");
--
2.43.0
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