[Qemu-devel] Re: [PATCH v2 1/2] block: Fix early failure in multiwrite

[Stefan Hajnoczi <stefanha@gmail.com>]

On Fri, Jul 2, 2010 at 1:07 PM, Kevin Wolf <kwolf@redhat.com> wrote:
> bdrv_aio_writev may call the callback immediately (and it will commonly do so
> in error cases). Current code doesn't consider this. For details see the
> comment added by this patch.
>
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> ---
>  block.c |   35 +++++++++++++++++++++++++++++------
>  1 files changed, 29 insertions(+), 6 deletions(-)
>
> diff --git a/block.c b/block.c
> index 9176dec..e65971c 100644
> --- a/block.c
> +++ b/block.c
> @@ -2183,8 +2183,29 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
>     // Check for mergable requests
>     num_reqs = multiwrite_merge(bs, reqs, num_reqs, mcb);
>
> -    // Run the aio requests
> +    /*
> +     * Run the aio requests. As soon as one request can't be submitted
> +     * successfully, fail all requests that are not yet submitted (we must
> +     * return failure for all requests anyway)
> +     *
> +     * num_requests cannot be set to the right value immediately: If
> +     * bdrv_aio_writev fails for some request, num_requests would be too high
> +     * and therefore multiwrite_cb() would never recognize the multiwrite
> +     * request as completed. We also cannot use the loop variable i to set it
> +     * when the first request fails because the callback may already have been
> +     * called for previously submitted requests. Thus, num_requests must be
> +     * incremented for each request that is submitted.
> +     *
> +     * The problem that callbacks may be called early also means that we need
> +     * to take care that num_requests doesn't become 0 before all requests are
> +     * submitted - multiwrite_cb() would consider the multiwrite request
> +     * completed. A dummy request that is "completed" by a manual call to
> +     * multiwrite_cb() takes care of this.
> +     */
> +    mcb->num_requests = 1;
> +
>     for (i = 0; i < num_reqs; i++) {
> +        mcb->num_requests++;
>         acb = bdrv_aio_writev(bs, reqs[i].sector, reqs[i].qiov,
>             reqs[i].nb_sectors, multiwrite_cb, mcb);
>
> @@ -2192,22 +2213,24 @@ int bdrv_aio_multiwrite(BlockDriverState *bs, BlockRequest *reqs, int num_reqs)
>             // We can only fail the whole thing if no request has been
>             // submitted yet. Otherwise we'll wait for the submitted AIOs to
>             // complete and report the error in the callback.
> -            if (mcb->num_requests == 0) {
> -                reqs[i].error = -EIO;
> +            if (i == 0) {
>                 goto fail;
>             } else {
> -                mcb->num_requests++;
>                 multiwrite_cb(mcb, -EIO);

When bdrv_aio_writev() fails we don't know if the callback has been
invoked by the block driver.  Qcow2 will invoke the callback in some
cases.  This is a problem because num_requests will be decremented
twice if we unconditionally call it here.

Stefan