Re: [CRIU] BUG: Dentry ffff9f795a08fe60{i=af565f, n=lo} still in use (1) [unmount of proc proc]

[Andrei Vagin <avagin@gmail.com>]

On Thu, Jul 06, 2017 at 08:41:00AM -0500, Eric W. Biederman wrote:
> Andrei Vagin <avagin@gmail.com> writes:
> 
> > I did a few experiments and found that the bug is reproduced for 6-12
> > hours on the our test server. Then I reverted two patches and the server
> > is working normally for more than 24 hours already, so the bug is
> > probably in one of these patches.
> >
> > commit e3d0065ab8535cbeee69a4c46a59f4d7360803ae
> > Author: Andrei Vagin <avagin@openvz.org>
> > Date:   Sun Jul 2 07:41:25 2017 +0200
> >
> >     Revert "proc/sysctl: prune stale dentries during unregistering"
> >     
> >     This reverts commit d6cffbbe9a7e51eb705182965a189457c17ba8a3.
> >
> > commit 2d3c50dac81011c1da4d2f7a63b84bd75287e320
> > Author: Andrei Vagin <avagin@openvz.org>
> > Date:   Sun Jul 2 07:40:08 2017 +0200
> >
> >     Revert "proc/sysctl: Don't grab i_lock under sysctl_lock."
> >     
> >     This reverts commit ace0c791e6c3cf5ef37cad2df69f0d90ccc40ffb.
> >
> >
> > FYI: This bug has been reproduced on 4.11.7
> 
> Instead of the revert could you test the patch below?

Our CI server are working with this patch for more than one day without
any problem.

Tested-by: Andrei Vagin <avagin@openvz.org>

Thanks,
Andrei
> 
> This should fix the issue by grabbing a s_active reference
> to the proc super block for every inode we flush.
> 
> diff --git a/fs/proc/internal.h b/fs/proc/internal.h
> index c5ae09b6c726..18694598bebf 100644
> --- a/fs/proc/internal.h
> +++ b/fs/proc/internal.h
> @@ -67,7 +67,7 @@ struct proc_inode {
>  	struct proc_dir_entry *pde;
>  	struct ctl_table_header *sysctl;
>  	struct ctl_table *sysctl_entry;
> -	struct list_head sysctl_inodes;
> +	struct hlist_node sysctl_inodes;
>  	const struct proc_ns_operations *ns_ops;
>  	struct inode vfs_inode;
>  };
> diff --git a/fs/proc/proc_sysctl.c b/fs/proc/proc_sysctl.c
> index 67985a7233c2..9bf06e2b1284 100644
> --- a/fs/proc/proc_sysctl.c
> +++ b/fs/proc/proc_sysctl.c
> @@ -191,7 +191,7 @@ static void init_header(struct ctl_table_header *head,
>  	head->set = set;
>  	head->parent = NULL;
>  	head->node = node;
> -	INIT_LIST_HEAD(&head->inodes);
> +	INIT_HLIST_HEAD(&head->inodes);
>  	if (node) {
>  		struct ctl_table *entry;
>  		for (entry = table; entry->procname; entry++, node++)
> @@ -261,25 +261,42 @@ static void unuse_table(struct ctl_table_header *p)
>  			complete(p->unregistering);
>  }
>  
> -/* called under sysctl_lock */
>  static void proc_sys_prune_dcache(struct ctl_table_header *head)
>  {
> -	struct inode *inode, *prev = NULL;
> +	struct inode *inode;
>  	struct proc_inode *ei;
> +	struct hlist_node *node;
> +	struct super_block *sb;
>  
>  	rcu_read_lock();
> -	list_for_each_entry_rcu(ei, &head->inodes, sysctl_inodes) {
> -		inode = igrab(&ei->vfs_inode);
> -		if (inode) {
> -			rcu_read_unlock();
> -			iput(prev);
> -			prev = inode;
> -			d_prune_aliases(inode);
> +	for (;;) {
> +		node = hlist_first_rcu(&head->inodes);
> +		if (!node)
> +			break;
> +		ei = hlist_entry(node, struct proc_inode, sysctl_inodes);
> +		spin_lock(&sysctl_lock);
> +		hlist_del_init_rcu(&ei->sysctl_inodes);
> +		spin_unlock(&sysctl_lock);
> +
> +		inode = &ei->vfs_inode;
> +		sb = inode->i_sb;
> +		if (!atomic_inc_not_zero(&sb->s_active))
> +			continue;
> +		inode = igrab(inode);
> +		rcu_read_unlock();
> +		if (unlikely(!inode)) {
> +			deactivate_super(sb);
>  			rcu_read_lock();
> +			continue;
>  		}
> +
> +		d_prune_aliases(inode);
> +		iput(inode);
> +		deactivate_super(sb);
> +
> +		rcu_read_lock();
>  	}
>  	rcu_read_unlock();
> -	iput(prev);
>  }
>  
>  /* called under sysctl_lock, will reacquire if has to wait */
> @@ -461,7 +478,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  	}
>  	ei->sysctl = head;
>  	ei->sysctl_entry = table;
> -	list_add_rcu(&ei->sysctl_inodes, &head->inodes);
> +	hlist_add_head_rcu(&ei->sysctl_inodes, &head->inodes);
>  	head->count++;
>  	spin_unlock(&sysctl_lock);
>  
> @@ -489,7 +506,7 @@ static struct inode *proc_sys_make_inode(struct super_block *sb,
>  void proc_sys_evict_inode(struct inode *inode, struct ctl_table_header *head)
>  {
>  	spin_lock(&sysctl_lock);
> -	list_del_rcu(&PROC_I(inode)->sysctl_inodes);
> +	hlist_del_init_rcu(&PROC_I(inode)->sysctl_inodes);
>  	if (!--head->count)
>  		kfree_rcu(head, rcu);
>  	spin_unlock(&sysctl_lock);
> diff --git a/include/linux/sysctl.h b/include/linux/sysctl.h
> index 80d07816def0..1c04a26bfd2f 100644
> --- a/include/linux/sysctl.h
> +++ b/include/linux/sysctl.h
> @@ -143,7 +143,7 @@ struct ctl_table_header
>  	struct ctl_table_set *set;
>  	struct ctl_dir *parent;
>  	struct ctl_node *node;
> -	struct list_head inodes; /* head for proc_inode->sysctl_inodes */
> +	struct hlist_head inodes; /* head for proc_inode->sysctl_inodes */
>  };
>  
>  struct ctl_dir {
>