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

On Thu, Jul 30, 2026 at 04:40:00PM +0800, 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 <[email protected]>

Sorry for the delay, this slipped through the cracks:

Acked-by: Roger Pau Monné <[email protected]>

Thanks, Roger.