Re: [Qemu-devel] [PATCH v3 18/29] block: Make overlap range for serialisation dynamic

["Benoît Canet" <benoit.canet@irqsave.net>]

Le Friday 17 Jan 2014 à 15:15:08 (+0100), Kevin Wolf a écrit :
> Copy on Read wants to serialise with all requests touching the same
> cluster, so wait_serialising_requests() rounded to cluster boundaries.
> Other users like alignment RMW will have different requirements, though
> (requests touching the same sector), so make it dynamic.
> 
> Signed-off-by: Kevin Wolf <kwolf@redhat.com>
> Reviewed-by: Max Reitz <mreitz@redhat.com>
> ---
>  block.c                   | 53 ++++++++++++++++++++++++-----------------------
>  include/block/block_int.h |  4 ++++
>  2 files changed, 31 insertions(+), 26 deletions(-)
> 
> diff --git a/block.c b/block.c
> index efa8979..e72966a 100644
> --- a/block.c
> +++ b/block.c
> @@ -2051,6 +2051,8 @@ static void tracked_request_begin(BdrvTrackedRequest *req,
>          .is_write       = is_write,
>          .co             = qemu_coroutine_self(),
>          .serialising    = false,
> +        .overlap_offset = offset,
> +        .overlap_bytes  = bytes,
>      };
>  
>      qemu_co_queue_init(&req->wait_queue);
> @@ -2058,12 +2060,19 @@ static void tracked_request_begin(BdrvTrackedRequest *req,
>      QLIST_INSERT_HEAD(&bs->tracked_requests, req, list);
>  }
>  
> -static void mark_request_serialising(BdrvTrackedRequest *req)
> +static void mark_request_serialising(BdrvTrackedRequest *req, size_t align)
>  {
> +    int64_t overlap_offset = req->offset & ~(align - 1);
> +    int overlap_bytes = ROUND_UP(req->offset + req->bytes, align)
> +                      - overlap_offset;
> +
>      if (!req->serialising) {
>          req->bs->serialising_in_flight++;
>          req->serialising = true;
>      }
> +
> +    req->overlap_offset = MIN(req->overlap_offset, overlap_offset);
> +    req->overlap_bytes = MAX(req->overlap_bytes, overlap_bytes);
>  }
>  
>  /**
> @@ -2087,20 +2096,16 @@ void bdrv_round_to_clusters(BlockDriverState *bs,
>      }
>  }
>  
> -static void round_bytes_to_clusters(BlockDriverState *bs,
> -                                    int64_t offset, unsigned int bytes,
> -                                    int64_t *cluster_offset,
> -                                    unsigned int *cluster_bytes)
> +static int bdrv_get_cluster_size(BlockDriverState *bs)
>  {
>      BlockDriverInfo bdi;
> +    int ret;
>  
> -    if (bdrv_get_info(bs, &bdi) < 0 || bdi.cluster_size == 0) {
> -        *cluster_offset = offset;
> -        *cluster_bytes = bytes;
> +    ret = bdrv_get_info(bs, &bdi);
> +    if (ret < 0 || bdi.cluster_size == 0) {
> +        return bs->request_alignment;
>      } else {
> -        *cluster_offset = QEMU_ALIGN_DOWN(offset, bdi.cluster_size);
> -        *cluster_bytes = QEMU_ALIGN_UP(offset - *cluster_offset + bytes,
> -                                       bdi.cluster_size);
> +        return bdi.cluster_size;
>      }
>  }
>  
> @@ -2108,11 +2113,11 @@ static bool tracked_request_overlaps(BdrvTrackedRequest *req,
>                                       int64_t offset, unsigned int bytes)
>  {
>      /*        aaaa   bbbb */
> -    if (offset >= req->offset + req->bytes) {
> +    if (offset >= req->overlap_offset + req->overlap_bytes) {
>          return false;
>      }
>      /* bbbb   aaaa        */
> -    if (req->offset >= offset + bytes) {
> +    if (req->overlap_offset >= offset + bytes) {
>          return false;
>      }
>      return true;
> @@ -2122,30 +2127,21 @@ static void coroutine_fn wait_serialising_requests(BdrvTrackedRequest *self)
>  {
>      BlockDriverState *bs = self->bs;
>      BdrvTrackedRequest *req;
> -    int64_t cluster_offset;
> -    unsigned int cluster_bytes;
>      bool retry;
>  
>      if (!bs->serialising_in_flight) {
>          return;
>      }
>  
> -    /* If we touch the same cluster it counts as an overlap.  This guarantees
> -     * that allocating writes will be serialized and not race with each other
> -     * for the same cluster.  For example, in copy-on-read it ensures that the
> -     * CoR read and write operations are atomic and guest writes cannot
> -     * interleave between them.
> -     */
> -    round_bytes_to_clusters(bs, self->offset, self->bytes,
> -                            &cluster_offset, &cluster_bytes);
> -
>      do {
>          retry = false;
>          QLIST_FOREACH(req, &bs->tracked_requests, list) {
>              if (req == self || (!req->serialising && !self->serialising)) {
>                  continue;
>              }
> -            if (tracked_request_overlaps(req, cluster_offset, cluster_bytes)) {
> +            if (tracked_request_overlaps(req, self->overlap_offset,
> +                                         self->overlap_bytes))
> +            {
>                  /* Hitting this means there was a reentrant request, for
>                   * example, a block driver issuing nested requests.  This must
>                   * never happen since it means deadlock.
> @@ -2761,7 +2757,12 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
>  
>      /* Handle Copy on Read and associated serialisation */
>      if (flags & BDRV_REQ_COPY_ON_READ) {
> -        mark_request_serialising(req);
> +        /* If we touch the same cluster it counts as an overlap.  This
> +         * guarantees that allocating writes will be serialized and not race
> +         * with each other for the same cluster.  For example, in copy-on-read
> +         * it ensures that the CoR read and write operations are atomic and
> +         * guest writes cannot interleave between them. */
> +        mark_request_serialising(req, bdrv_get_cluster_size(bs));
>      }
>  
>      wait_serialising_requests(req);
> diff --git a/include/block/block_int.h b/include/block/block_int.h
> index d8443df..ccd2c68 100644
> --- a/include/block/block_int.h
> +++ b/include/block/block_int.h
> @@ -60,7 +60,11 @@ typedef struct BdrvTrackedRequest {
>      int64_t offset;
>      unsigned int bytes;
>      bool is_write;
> +
>      bool serialising;
> +    int64_t overlap_offset;
> +    unsigned int overlap_bytes;
> +
>      QLIST_ENTRY(BdrvTrackedRequest) list;
>      Coroutine *co; /* owner, used for deadlock detection */
>      CoQueue wait_queue; /* coroutines blocked on this request */
> -- 
> 1.8.1.4
> 
> 
Reviewed-by: Benoit Canet <benoit@irqsave.net>