Re: [Qemu-devel] [RFC V6 25/33] qcow2: Serialize write requests when deduplication is activated.

[Stefan Hajnoczi]

On Wed, Feb 06, 2013 at 01:31:58PM +0100, Benoît Canet wrote:
> This fix the sub cluster sized writes race conditions while waiting
> for a more faster solution.
> 
> Signed-off-by: Benoit Canet <address@hidden>
> ---
>  block/qcow2.c |   14 +++++++++++++-
>  block/qcow2.h |    1 +
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/block/qcow2.c b/block/qcow2.c
> index f39b6d5..5cfffd2 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -524,6 +524,7 @@ static int qcow2_open(BlockDriverState *bs, int flags)
>  
>      /* Initialise locks */
>      qemu_co_mutex_init(&s->lock);
> +    qemu_co_mutex_init(&s->dedup_lock);
>  
>      /* Repair image if dirty */
>      if (!(flags & BDRV_O_CHECK) && !bs->read_only &&
> @@ -815,8 +816,15 @@ static coroutine_fn int qcow2_co_writev(BlockDriverState 
> *bs,
>      s->cluster_cache_offset = -1; /* disable compressed cache */
>  
>      qemu_co_mutex_lock(&s->lock);
> -
>      atomic_dedup_is_running = qcow2_dedup_is_running(bs);
> +    qemu_co_mutex_unlock(&s->lock);

No need to lock/unlock around qcow2_dedup_is_running(bs) because it
doesn't yield.  No other coroutine can run while our thread of control
is active here.

> +
> +    if (atomic_dedup_is_running) {
> +        qemu_co_mutex_lock(&s->dedup_lock);
> +    }

What exactly does dedup_lock protect?  Please document it.

> +
> +    qemu_co_mutex_lock(&s->lock);
> +
>      if (atomic_dedup_is_running) {
>          QTAILQ_INIT(&ds.undedupables);
>          ds.phash.reuse = false;
> @@ -983,6 +991,10 @@ fail:
>          g_free(l2meta);
>      }
>  
> +    if (atomic_dedup_is_running) {
> +        qemu_co_mutex_unlock(&s->dedup_lock);
> +    }
> +
>      qemu_iovec_destroy(&hd_qiov);
>      qemu_vfree(cluster_data);
>      qemu_vfree(dedup_cluster_data);
> diff --git a/block/qcow2.h b/block/qcow2.h
> index a13ec75..81dc0ea 100644
> --- a/block/qcow2.h
> +++ b/block/qcow2.h
> @@ -236,6 +236,7 @@ typedef struct BDRVQcowState {
>      GTree *dedup_tree_by_sect;
>  
>      CoMutex lock;
> +    CoMutex dedup_lock;
>  
>      uint32_t crypt_method; /* current crypt method, 0 if no key yet */
>      uint32_t crypt_method_header;
> -- 
> 1.7.10.4
>