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[PATCH] domain: add barrier in vcpu_create()
- To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 26 Nov 2025 11:13:37 +0100
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Michal Orzel <michal.orzel@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>, Bertrand Marquis <bertrand.marquis@xxxxxxx>
- Delivery-date: Wed, 26 Nov 2025 10:13:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
The lock-less list updating isn't safe against racing for_each_vcpu(),
unless done (by hardware) in exactly the order written.
Fixes: 3037c5a2cb82 ("arm: domain")
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
The Fixes: tag is pretty arbitrary; the issue was becoming non-latent when
Arm support was added. (Strictly speaking IA-64 and PPC would have been
affected too afaict, just that now that doesn't matter anymore [or, for
PPC, not yet, seeing that its support is being re-built from scratch].)
I'm not quite happy about prev_id being plain int, but changing it to
unsigned (with suitable other adjustments) actually makes gcc15 generate
worse code on x86.
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -464,6 +464,7 @@ struct vcpu *vcpu_create(struct domain *
prev_id--;
BUG_ON(prev_id < 0);
v->next_in_list = d->vcpu[prev_id]->next_in_list;
+ smp_wmb();
d->vcpu[prev_id]->next_in_list = v;
}
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