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Date: Fri, 28 Aug 2026 11:37:48 +0200
From: Roger Pau =?utf-8?B?TW9ubsOp?= <roger@xenproject.org>
To: Gui-Dong Han <hanguidong02@gmail.com>
Cc: xen-devel@lists.xenproject.org, axboe@kernel.dk,
	linux-block@vger.kernel.org, konrad.wilk@oracle.com,
	linux-kernel@vger.kernel.org, baijiaju1990@gmail.com
Subject: Re: [PATCH] xen/blkback: Prevent missed completion when draining I/O
Message-ID: <apFW7CH1aZasMKiV@macbook.local>
References: <20260730084000.3305702-1-hanguidong02@gmail.com>
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In-Reply-To: <20260730084000.3305702-1-hanguidong02@gmail.com>

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 <hanguidong02@gmail.com>

Sorry for the delay, this slipped through the cracks:

Acked-by: Roger Pau Monné <roger@xenproject.org>

Thanks, Roger.