Re: [PATCH v2] xen/arm64/atomic: Clear exclusive monitor on cmpxchg failure

[Julien Grall <julien@xxxxxxx>]

Hi Ryoji,

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.
The Xen atomics operations were originally taken from Linux. Looking at 
the implementation there, I don't see a clrex on the failure path. Do 
you have more details why we would want it?
Also, if this is necessary on arm64, then we most likely we want the 
same for the arm32 implementation (including __atomic_add_unless()).
        

Cheers,

--
Julien Grall