Date: Tue, 4 Nov 2025 14:36:09 GMT From: Mark Johnston <markj@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-main@FreeBSD.org Subject: git: f999ffdce381 - main - virtio: Fix polling in virtqueue_dequeue() Message-ID: <202511041436.5A4Ea9x8004926@gitrepo.freebsd.org>
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The branch main has been updated by markj: URL: https://cgit.FreeBSD.org/src/commit/?id=f999ffdce3813eb946f10999ccffb8275c324469 commit f999ffdce3813eb946f10999ccffb8275c324469 Author: Mark Johnston <markj@FreeBSD.org> AuthorDate: 2025-11-04 14:27:33 +0000 Commit: Mark Johnston <markj@FreeBSD.org> CommitDate: 2025-11-04 14:35:31 +0000 virtio: Fix polling in virtqueue_dequeue() The access of vq->vq_ring.used->idx needs to be volatile-qualified, otherwise the compiler may optimize virtqueue_poll() into an infinite loop if there is no data available upon the first poll. Prior to commit ad17789a8569 this wasn't a problem since an external function call after each poll inhibited the optimization. PR: 289930 MFC after: 3 days Sponsored by: Klara, Inc. Fixes: ad17789a8569 ("virtio: Remove the unused poll method") --- sys/dev/virtio/virtqueue.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/sys/dev/virtio/virtqueue.c b/sys/dev/virtio/virtqueue.c index cc7a233d60ee..41e01549c8b2 100644 --- a/sys/dev/virtio/virtqueue.c +++ b/sys/dev/virtio/virtqueue.c @@ -580,7 +580,8 @@ virtqueue_dequeue(struct virtqueue *vq, uint32_t *len) void *cookie; uint16_t used_idx, desc_idx; - if (vq->vq_used_cons_idx == vq_htog16(vq, vq->vq_ring.used->idx)) + if (vq->vq_used_cons_idx == + vq_htog16(vq, atomic_load_16(&vq->vq_ring.used->idx))) return (NULL); used_idx = vq->vq_used_cons_idx++ & (vq->vq_nentries - 1);
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