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SubjectRe: [PATCH 1/3] xen/blkback: Support 'feature-barrier' aka old-style BARRIER requests.
>>> On 10.10.11 at 17:28, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> We emulate the barrier requests by draining the outstanding bio's
> and then sending the WRITE_FLUSH command. To drain the I/Os
> we use the refcnt that is used during disconnect to wait for all
> the I/Os before disconnecting from the frontend. We latch on its
> value and if it reaches either the threshold for disconnect or when
> there are no more outstanding I/Os, then we have drained all I/Os.
>
> Suggested-by: Christopher Hellwig <hch@infradead.org>
> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> ---
> drivers/block/xen-blkback/blkback.c | 37 +++++++++++++++++++++++++++++++++-
> drivers/block/xen-blkback/common.h | 5 ++++
> drivers/block/xen-blkback/xenbus.c | 18 +++++++++++++++++
> 3 files changed, 58 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/block/xen-blkback/blkback.c
> b/drivers/block/xen-blkback/blkback.c
> index e0dab61..184b133 100644
> --- a/drivers/block/xen-blkback/blkback.c
> +++ b/drivers/block/xen-blkback/blkback.c
> @@ -452,6 +452,23 @@ static void xen_blk_discard(struct xen_blkif *blkif,
> struct blkif_request *req)
> make_response(blkif, req->id, req->operation, status);
> }
>
> +static void xen_blk_drain_io(struct xen_blkif *blkif)
> +{
> + atomic_set(&blkif->drain, 1);
> + do {
> + wait_for_completion_interruptible_timeout(
> + &blkif->drain_complete, HZ);
> +
> + if (!atomic_read(&blkif->drain))
> + break;
> + /* The initial value is one, and one refcnt taken at the
> + * start of the xen_blkif_schedule thread. */
> + if (atomic_read(&blkif->refcnt) <= 2)
> + break;

Shouldn't this test be done the very first thing in the loop? It looks
racy the way it's placed now, and it would incur a 1 sec stall if this
was the only request currently being processed (as no completion
of ane earlier request could signal completion).

Jan

> + } while (!kthread_should_stop());
> + atomic_set(&blkif->drain, 0);
> +}




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