Re: [Qemu-devel] [PATCH] block/vdi: Fix locking for parallel requests

[Paolo Bonzini <pbonzini@redhat.com>]

Cc: qemu-stable <qemu-stable@nongnu.org>

On 17/02/2015 22:33, Max Reitz wrote:
> Concurrently modifying the bmap is not a good idea; this patch adds a
> lock for it. See https://bugs.launchpad.net/qemu/+bug/1422307 for what
> can go wrong without.
> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/vdi.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/block/vdi.c b/block/vdi.c
> index 74030c6..c5ff428 100644
> --- a/block/vdi.c
> +++ b/block/vdi.c
> @@ -51,6 +51,7 @@
>  
>  #include "qemu-common.h"
>  #include "block/block_int.h"
> +#include "block/coroutine.h"
>  #include "qemu/module.h"
>  #include "migration/migration.h"
>  
> @@ -196,6 +197,8 @@ typedef struct {
>      /* VDI header (converted to host endianness). */
>      VdiHeader header;
>  
> +    CoMutex bmap_lock;
> +
>      Error *migration_blocker;
>  } BDRVVdiState;
>  
> @@ -498,6 +501,8 @@ static int vdi_open(BlockDriverState *bs, QDict *options, int flags,
>          goto fail_free_bmap;
>      }
>  
> +    qemu_co_mutex_init(&s->bmap_lock);
> +
>      /* Disable migration when vdi images are used */
>      error_set(&s->migration_blocker,
>                QERR_BLOCK_FORMAT_FEATURE_NOT_SUPPORTED,
> @@ -619,6 +624,9 @@ static int vdi_co_write(BlockDriverState *bs,
>                 n_sectors, sector_num);
>  
>          /* prepare next AIO request */
> +        if (!block) {
> +            qemu_co_mutex_lock(&s->bmap_lock);
> +        }
>          bmap_entry = le32_to_cpu(s->bmap[block_index]);
>          if (!VDI_IS_ALLOCATED(bmap_entry)) {
>              /* Allocate new block and write to it. */
> @@ -641,9 +649,13 @@ static int vdi_co_write(BlockDriverState *bs,
>                     (s->block_sectors - n_sectors - sector_in_block) * SECTOR_SIZE);
>              ret = bdrv_write(bs->file, offset, block, s->block_sectors);
>          } else {
> -            uint64_t offset = s->header.offset_data / SECTOR_SIZE +
> -                              (uint64_t)bmap_entry * s->block_sectors +
> -                              sector_in_block;
> +            uint64_t offset;
> +
> +            qemu_co_mutex_unlock(&s->bmap_lock);
> +
> +            offset = s->header.offset_data / SECTOR_SIZE +
> +                     (uint64_t)bmap_entry * s->block_sectors +
> +                     sector_in_block;
>              ret = bdrv_write(bs->file, offset, buf, n_sectors);
>          }
>  
> @@ -656,6 +668,9 @@ static int vdi_co_write(BlockDriverState *bs,
>  
>      logout("finished data write\n");
>      if (ret < 0) {
> +        if (block) {
> +            qemu_co_mutex_unlock(&s->bmap_lock);
> +        }
>          return ret;
>      }
>  
> @@ -688,6 +703,8 @@ static int vdi_co_write(BlockDriverState *bs,
>          logout("will write %u block map sectors starting from entry %u\n",
>                 n_sectors, bmap_first);
>          ret = bdrv_write(bs->file, offset, base, n_sectors);
> +
> +        qemu_co_mutex_unlock(&s->bmap_lock);
>      }
>  
>      return ret;
>