Re: [Qemu-devel] [RFC 1/6] block: add request tracking

[Kevin Wolf <kwolf@redhat.com>]

Am 17.10.2011 17:47, schrieb Stefan Hajnoczi:
> The block layer does not know about pending requests.  This information
> is necessary for copy-on-read since overlapping requests must be
> serialized to prevent races that corrupt the image.
> 
> Add a simple mechanism to enable/disable request tracking.  By default
> request tracking is disabled.
> 
> The BlockDriverState gets a new tracked_request list field which
> contains all pending requests.  Each request is a BdrvTrackedRequest
> record with sector_num, nb_sectors, and is_write fields.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
> ---
>  block.c     |   83 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  block_int.h |    8 +++++
>  2 files changed, 90 insertions(+), 1 deletions(-)
> 
> diff --git a/block.c b/block.c
> index 9873b57..2d2c62a 100644
> --- a/block.c
> +++ b/block.c
> @@ -978,6 +978,77 @@ void bdrv_commit_all(void)
>      }
>  }
>  
> +struct BdrvTrackedRequest {
> +    BlockDriverState *bs;
> +    int64_t sector_num;
> +    int nb_sectors;
> +    bool is_write;
> +    QLIST_ENTRY(BdrvTrackedRequest) list;
> +};
> +
> +/**
> + * Remove an active request from the tracked requests list
> + *
> + * If req is NULL, no operation is performed.
> + *
> + * This function should be called when a tracked request is completing.
> + */
> +static void tracked_request_remove(BdrvTrackedRequest *req)
> +{
> +    if (req) {
> +        QLIST_REMOVE(req, list);
> +        g_free(req);
> +    }
> +}
> +
> +/**
> + * Add an active request to the tracked requests list
> + *
> + * Return NULL if request tracking is disabled, non-NULL otherwise.
> + */
> +static BdrvTrackedRequest *tracked_request_add(BlockDriverState *bs,
> +                                               int64_t sector_num,
> +                                               int nb_sectors, bool is_write)
> +{
> +    BdrvTrackedRequest *req;
> +
> +    if (!bs->request_tracking) {
> +        return NULL;
> +    }
> +
> +    req = g_malloc(sizeof(*req));
> +    req->bs = bs;
> +    req->sector_num = sector_num;
> +    req->nb_sectors = nb_sectors;
> +    req->is_write = is_write;

How about using g_malloc0 or a compound literal assignment for
initialisation, so that we won't get surprises when the struct is extended?

> +
> +    QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
> +
> +    return req;
> +}
> +
> +/**
> + * Enable tracking of incoming requests
> + *
> + * Request tracking can be safely used by multiple users at the same time,
> + * there is an internal reference count to match start and stop calls.
> + */
> +void bdrv_start_request_tracking(BlockDriverState *bs)
> +{
> +    bs->request_tracking++;
> +}
> +
> +/**
> + * Disable tracking of incoming requests
> + *
> + * Note that in-flight requests are still tracked, this function only stops
> + * tracking incoming requests.
> + */
> +void bdrv_stop_request_tracking(BlockDriverState *bs)
> +{
> +    bs->request_tracking--;
> +}
> +
>  /*
>   * Return values:
>   * 0        - success
> @@ -1262,6 +1333,8 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
>      int64_t sector_num, int nb_sectors, QEMUIOVector *qiov)
>  {
>      BlockDriver *drv = bs->drv;
> +    BdrvTrackedRequest *req;
> +    int ret;
>  
>      if (!drv) {
>          return -ENOMEDIUM;
> @@ -1270,7 +1343,10 @@ static int coroutine_fn bdrv_co_do_readv(BlockDriverState *bs,
>          return -EIO;
>      }
>  
> -    return drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> +    req = tracked_request_add(bs, sector_num, nb_sectors, false);
> +    ret = drv->bdrv_co_readv(bs, sector_num, nb_sectors, qiov);
> +    tracked_request_remove(req);

Hm... Do we actually need to malloc the BdrvTrackedRequest? If all users
are like this (and at least those in this patch are), we can just keep
it on the coroutine stack.

Kevin