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Re: [PATCH] xen/blkback: Prevent missed completion when draining I/O
- To: Gui-Dong Han <hanguidong02@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 30 Jul 2026 10:43:43 +0200
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- Cc: axboe@xxxxxxxxx, linux-block@xxxxxxxxxxxxxxx, konrad.wilk@xxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, baijiaju1990@xxxxxxxxx, roger@xxxxxxxxxxxxxx, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 30 Jul 2026 08:43:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 30.07.2026 10:40, Gui-Dong Han wrote:
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -1021,6 +1021,13 @@ static void xen_blk_drain_io(struct xen_blkif_ring
> *ring)
> struct xen_blkif *blkif = ring->blkif;
>
> atomic_set(&blkif->drain, 1);
> + /*
> + * Publish drain before checking inflight. Otherwise,
> + * xen_blkbk_unmap_and_respond_callback() can decrement inflight with
> + * atomic_dec_and_test() and still see drain == 0 after this path saw
> + * inflight > 0, missing the completion.
> + */
> + smp_mb();
> do {
> if (atomic_read(&ring->inflight) == 0)
> break;
Yet then don't we also need a barrier in
xen_blkbk_unmap_and_respond_callback()'s
check? Barriers almost always come in pairs, after all.
Jan
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