Re: [PATCH] dm-table: delayed cleanup for dm_table_destroy()

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Hi Hannes

I wouldn't do this. This behavior actually existed in kernels <= 2.6.28.
And it caused trouble.

The problems are these:

Some code may hold a spinlock and call dm_table_put. Currently, 
dm_table_put just decrements the counter and the cleanup is done 
synchronously in dm_table_destroy. With your patch, cleanup would be done 
in dm_table_put --- you call a target destructor (which may sleep) in a 
non-sleeping context.

Some code may hold a mutex and call dm_table_put. If you call a target 
destructor from dm_table_put and it takes the same mutex, it deadlocks.

Currently, when dm_table_destroy exits, it is guaranteed that the table is 
destroyed and all target destructors have been called. With your patch, 
dm_table_destroy may exit and the table could be still alive because of 
some other reference. This may cause leaked references to some other files 
or devices. For example, suppose that the user has a device mapper device 
"dm" that redirects requests to "/dev/sda". The user assumes that if he 
runs "dmsetup remove dm" and this command returns, "/dev/sda" will not be 
open. Your patch breaks this assumption.

Look at the commit "d58168763f74d1edbc296d7038c60efe6493fdd4" in 2.6.29 
--- I was actually removing the old-style reference counting (that is 
functionally equivalent to what your patch does) because of these three 
problems. The old code really caused a crash although it happened very 
rarely.

Mikulas


On Mon, 19 Mar 2012, Hannes Reinecke wrote:

> We should be using a kref instead of the existing holders flag.
> This enables us to use delayed cleanup and we'll get rid of the
> msleep in dm_table_destroy().
> 
> Signed-off-by: Hannes Reinecke <hare@xxxxxxx>
> ---
>  drivers/md/dm-table.c |   26 +++++++++++++-------------
>  1 files changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index a3d1e18..97c01f7 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -41,7 +41,7 @@
>  
>  struct dm_table {
>  	struct mapped_device *md;
> -	atomic_t holders;
> +	struct kref kref;
>  	unsigned type;
>  
>  	/* btree table */
> @@ -208,7 +208,7 @@ int dm_table_create(struct dm_table **result, fmode_t mode,
>  
>  	INIT_LIST_HEAD(&t->devices);
>  	INIT_LIST_HEAD(&t->target_callbacks);
> -	atomic_set(&t->holders, 0);
> +	kref_init(&t->kref);
>  
>  	if (!num_targets)
>  		num_targets = KEYS_PER_NODE;
> @@ -240,17 +240,11 @@ static void free_devices(struct list_head *devices)
>  	}
>  }
>  
> -void dm_table_destroy(struct dm_table *t)
> +void __table_destroy(struct kref *kref)
>  {
> +	struct dm_table *t = container_of(kref, struct dm_table, kref);
>  	unsigned int i;
>  
> -	if (!t)
> -		return;
> -
> -	while (atomic_read(&t->holders))
> -		msleep(1);
> -	smp_mb();
> -
>  	/* free the indexes */
>  	if (t->depth >= 2)
>  		vfree(t->index[t->depth - 2]);
> @@ -277,7 +271,8 @@ void dm_table_destroy(struct dm_table *t)
>  
>  void dm_table_get(struct dm_table *t)
>  {
> -	atomic_inc(&t->holders);
> +	if (t)
> +		kref_get(&t->kref);
>  }
>  EXPORT_SYMBOL(dm_table_get);
>  
> @@ -286,11 +281,16 @@ void dm_table_put(struct dm_table *t)
>  	if (!t)
>  		return;
>  
> -	smp_mb__before_atomic_dec();
> -	atomic_dec(&t->holders);
> +	kref_put(&t->kref, __table_destroy);
>  }
>  EXPORT_SYMBOL(dm_table_put);
>  
> +void dm_table_destroy(struct dm_table *t)
> +{
> +	smp_mb__before_atomic_dec();
> +	dm_table_put(t);
> +}
> +
>  /*
>   * Checks to see if we need to extend highs or targets.
>   */
> -- 
> 1.6.0.2
> 
> --
> dm-devel mailing list
> dm-devel@xxxxxxxxxx
> https://www.redhat.com/mailman/listinfo/dm-devel
> 

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