Subject: Re: [PATCH net-next v4] rhashtable: Fix race in rhashtable_destroy() and use regular work_struct

From: Ying Xue <[email-masked]>
Date: Fri, 16 Jan 2015 11:08:42 +0800

On 01/16/2015 10:23 AM, Ying Xue wrote:
> When we put our declared work task in the global workqueue with
> schedule_delayed_work(), its delay parameter is always zero.
> Therefore, we should define a regular work in rhashtable structure
> instead of a delayed work.
> 
> By the way, we add a condition to check whether resizing functions
> are NULL before cancelling the work, avoiding to cancel an
> uninitialized work.
> 
> Lastly, while we wait for all work items we submitted before to run
> to completion with cancel_delayed_work(), ht->mutex has been taken in
> rhashtable_destroy(). Moreover, cancel_delayed_work() doesn't return
> until all work items are accomplished, and when work items are
> scheduled, the work's function - rht_deferred_worker() will be called.
> However, as rht_deferred_worker() also needs to acquire the lock,
> deadlock might happen at the moment as the lock is already held before.
> So if the cancel work function is moved out of the lock covered scope,
> this will avoid the deadlock.
> 
> Fixes: 97defe1 ("rhashtable: Per bucket locks & deferred expansion/shrinking")
> Signed-off-by: Ying Xue <[email-masked]>
> Cc: Thomas Graf <[email-masked]>
> Signed-off-by: Ying Xue <[email-masked]>

Sorry, there is a duplicated my "signed-off" but lack of Thomas's
"acked-by". So please ignore the version.

Regards,
Ying

> ---
> Change log:
> v4:
>   - Add an empty line in rhashtable_destroy()
>   - Revise commit description
> v3:
>   - Revise patch header description from Thomas's suggestion
> v2:
>   - Move cancel_work_sync() out of ht->mutex lock scope from Thomas's
>     comments
>  include/linux/rhashtable.h |    2 +-
>  lib/rhashtable.c           |   12 ++++++------
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/rhashtable.h b/include/linux/rhashtable.h
> index 9570832..a2562ed 100644
> --- a/include/linux/rhashtable.h
> +++ b/include/linux/rhashtable.h
> @@ -119,7 +119,7 @@ struct rhashtable {
>  	atomic_t			nelems;
>  	atomic_t			shift;
>  	struct rhashtable_params	p;
> -	struct delayed_work             run_work;
> +	struct work_struct		run_work;
>  	struct mutex                    mutex;
>  	bool                            being_destroyed;
>  };
> diff --git a/lib/rhashtable.c b/lib/rhashtable.c
> index aca6998..84a78e3 100644
> --- a/lib/rhashtable.c
> +++ b/lib/rhashtable.c
> @@ -485,7 +485,7 @@ static void rht_deferred_worker(struct work_struct *work)
>  	struct rhashtable *ht;
>  	struct bucket_table *tbl;
>  
> -	ht = container_of(work, struct rhashtable, run_work.work);
> +	ht = container_of(work, struct rhashtable, run_work);
>  	mutex_lock(&ht->mutex);
>  	tbl = rht_dereference(ht->tbl, ht);
>  
> @@ -507,7 +507,7 @@ static void rhashtable_wakeup_worker(struct rhashtable *ht)
>  	if (tbl == new_tbl &&
>  	    ((ht->p.grow_decision && ht->p.grow_decision(ht, size)) ||
>  	     (ht->p.shrink_decision && ht->p.shrink_decision(ht, size))))
> -		schedule_delayed_work(&ht->run_work, 0);
> +		schedule_work(&ht->run_work);
>  }
>  
>  static void __rhashtable_insert(struct rhashtable *ht, struct rhash_head *obj,
> @@ -903,7 +903,7 @@ int rhashtable_init(struct rhashtable *ht, struct rhashtable_params *params)
>  		get_random_bytes(&ht->p.hash_rnd, sizeof(ht->p.hash_rnd));
>  
>  	if (ht->p.grow_decision || ht->p.shrink_decision)
> -		INIT_DEFERRABLE_WORK(&ht->run_work, rht_deferred_worker);
> +		INIT_WORK(&ht->run_work, rht_deferred_worker);
>  
>  	return 0;
>  }
> @@ -921,11 +921,11 @@ void rhashtable_destroy(struct rhashtable *ht)
>  {
>  	ht->being_destroyed = true;
>  
> -	mutex_lock(&ht->mutex);
> +	if (ht->p.grow_decision || ht->p.shrink_decision)
> +		cancel_work_sync(&ht->run_work);
>  
> -	cancel_delayed_work(&ht->run_work);
> +	mutex_lock(&ht->mutex);
>  	bucket_table_free(rht_dereference(ht->tbl, ht));
> -
>  	mutex_unlock(&ht->mutex);
>  }
>  EXPORT_SYMBOL_GPL(rhashtable_destroy);
>