Re: [PATCH] xen/blkback: Prevent missed completion when draining I/O

[Juergen Gross]

On 30.07.26 10:40, Gui-Dong Han wrote:
> xen_blk_drain_io() sets drain before checking inflight.
> xen_blkbk_unmap_and_respond_callback() decrements inflight with
> atomic_dec_and_test() before checking drain.
>
> The pre-wait condition check was added to avoid missing a completion, but
> atomic_set() is unordered. With one I/O in flight, the drain path can set
> drain to 1 and read inflight as 1 before the completion decrement. The
> completion path then decrements inflight to 0 but can still read drain as
> 0. It skips complete(), so the drain path waits until the timeout despite
> no I/O remaining.
>
> Add a full barrier between setting drain and reading inflight.
> atomic_dec_and_test() already provides full ordering on the completion
> side.
>
> Fixes: 6927d92091df ("xen/blkback: Fix two races in the handling of barrier requests.")
> Signed-off-by: Gui-Dong Han <hanguidong02@xxxxxxxxx>

Reviewed-by: Juergen Gross <jgross@xxxxxxxx>


Juergen

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