linux-kernel - Re: [PATCH v2 7/8] shift percpu_counter_destroy() into destroy_super

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Date:	Thu, 13 Aug 2015 12:35:18 +0200
From:	Jan Kara <jack@...e.cz>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Dave Chinner <david@...morbit.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Jan Kara <jack@...e.cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/8] shift percpu_counter_destroy() into
 destroy_super_work()

On Tue 11-08-15 19:04:16, Oleg Nesterov wrote:
> Of course, this patch is ugly as hell. It will be (partially)
> reverted later. We add it to ensure that other WIP changes in
> percpu_rw_semaphore won't break fs/super.c.
> 
> We do not even need this change right now, percpu_free_rwsem()
> is fine in atomic context. But we are going to change this, it
> will be might_sleep() after we merge the rcu_sync() patches.
> 
> And even after that we do not really need destroy_super_work(),
> we will kill it in any case. Instead, destroy_super_rcu() should
> just check that rss->cb_state == CB_IDLE and do call_rcu() again
> in the (very unlikely) case this is not true.
> 
> So this is just the temporary kludge which helps us to avoid the
> conflicts with the changes which will be (hopefully) routed via
> rcu tree.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Looking into this again, it would seem somewhat cleaner to me to move the
destruction to deactivate_locked_super() instead. We already do this for
list_lru_destroy() because that can sleep as well and so I'd prefer to keep
these two things together. You have to be somewhat careful with the failure
path in alloc_super() but that's easily doable...

								Honza

> ---
>  fs/super.c         |   23 +++++++++++++++++++----
>  include/linux/fs.h |    3 ++-
>  2 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 89b58fb..75436e2 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -135,6 +135,24 @@ static unsigned long super_cache_count(struct shrinker *shrink,
>  	return total_objects;
>  }
>  
> +static void destroy_super_work(struct work_struct *work)
> +{
> +	struct super_block *s = container_of(work, struct super_block,
> +							destroy_work);
> +	int i;
> +
> +	for (i = 0; i < SB_FREEZE_LEVELS; i++)
> +		percpu_counter_destroy(&s->s_writers.counter[i]);
> +	kfree(s);
> +}
> +
> +static void destroy_super_rcu(struct rcu_head *head)
> +{
> +	struct super_block *s = container_of(head, struct super_block, rcu);
> +	INIT_WORK(&s->destroy_work, destroy_super_work);
> +	schedule_work(&s->destroy_work);
> +}
> +
>  /**
>   *	destroy_super	-	frees a superblock
>   *	@s: superblock to free
> @@ -143,16 +161,13 @@ static unsigned long super_cache_count(struct shrinker *shrink,
>   */
>  static void destroy_super(struct super_block *s)
>  {
> -	int i;
>  	list_lru_destroy(&s->s_dentry_lru);
>  	list_lru_destroy(&s->s_inode_lru);
> -	for (i = 0; i < SB_FREEZE_LEVELS; i++)
> -		percpu_counter_destroy(&s->s_writers.counter[i]);
>  	security_sb_free(s);
>  	WARN_ON(!list_empty(&s->s_mounts));
>  	kfree(s->s_subtype);
>  	kfree(s->s_options);
> -	kfree_rcu(s, rcu);
> +	call_rcu(&s->rcu, destroy_super_rcu);
>  }
>  
>  /**
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 78ac768..6addccc 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -30,6 +30,7 @@
>  #include <linux/lockdep.h>
>  #include <linux/percpu-rwsem.h>
>  #include <linux/blk_types.h>
> +#include <linux/workqueue.h>
>  
>  #include <asm/byteorder.h>
>  #include <uapi/linux/fs.h>
> @@ -1346,7 +1347,7 @@ struct super_block {
>  	struct list_lru		s_dentry_lru ____cacheline_aligned_in_smp;
>  	struct list_lru		s_inode_lru ____cacheline_aligned_in_smp;
>  	struct rcu_head		rcu;
> -
> +	struct work_struct	destroy_work;
>  	/*
>  	 * Indicates how deep in a filesystem stack this SB is
>  	 */
> -- 
> 1.5.5.1
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR
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