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Re: [PATCH v2] xen/arm64/atomic: Clear exclusive monitor on cmpxchg failure
- To: Ryoji Okamoto <okamoto@xxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 9 Sep 2026 08:31:00 +0200
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- Cc: Hirokazu Takahashi <taka@xxxxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 09 Sep 2026 06:31:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 09.09.2026 08:12, Ryoji Okamoto wrote:
> 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>
> ---
> v2: Use tabs for indentation instead of spaces
Well, ...
> --- 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"
... you now do here, but ...
> +"2: clrex\n"
... still not here.
Jan
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