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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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