[dm-devel] Re: snapshot merging: change timeout to a sequence count

[Mike Snitzer]

On Mon, Dec 07 2009 at 10:52am -0500,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Mon, Dec 07 2009 at  8:19am -0500,
> Mikulas Patocka <mpatocka at redhat.com> wrote:
> 
> > Hi
> > 
> > This changes the timeout to a sequence count. And adds a comment.
> > 
> > Mikulas
> > 
> > ---
> > 
> > Avoit the timeout.
> > 
> > Use a sequence count to resolve the race. The count increases each time
> > an exception reallocation finishes. Use wait_event() to wait until the count
> > changes.
> > 
> > The chunk-reallocation logic is explained in the comment in the patch.
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> 
> Here is an updated patch that falls at the end of my snapshot-merge
> series here:
> http://people.redhat.com/msnitzer/patches/snapshot-merge/kernel/2.6.33/
> 
> ---
> 
> dm snapshot: change the snapshot reallocation timeout to a sequence count
> 
> Use a sequence count to resolve the race between I/O to chunks that are
> about to be merged.  The count increases each time an exception
> reallocation finishes.  Use wait_event() to wait until the count
> changes.
> 
> The chunk-reallocation logic is now explained in snapshot_merge_process()
> 
> Signed-off-by: Mikulas Patocka <mpatocka at redhat.com>
> ---
>  drivers/md/dm-snap.c |   45 +++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 43 insertions(+), 2 deletions(-)
> 
> Index: linux-rhel6/drivers/md/dm-snap.c
> ===================================================================
> --- linux-rhel6.orig/drivers/md/dm-snap.c
> +++ linux-rhel6/drivers/md/dm-snap.c
> @@ -271,6 +271,8 @@ static struct list_head *_origins;
>  static struct rw_semaphore _origins_lock;
>  
>  static DECLARE_WAIT_QUEUE_HEAD(_pending_exception_done);
> +static DEFINE_SPINLOCK(_pending_exception_done_spinlock);
> +static u64 _pending_exception_done_count = 0;

BTW, checkpatch issues the following error:

ERROR: do not initialise statics to 0 or NULL
#24: FILE: drivers/md/dm-snap.c:275:
+static u64 _pending_exception_done_count = 0;