[Ocfs2-devel] [PATCH 5/6] ocfs2: Avoid blocking in ocfs2_mark_lockres_freeing() in downconvert thread

[Srinivas Eeda <srinivas.eeda@oracle.com>]

I like the idea of dc_task handling queued basts in 
ocfs2_mark_lockres_freeing.

I am wondering if we should call lockres->l_ops->post_unlock(osb, 
lockres) ? Would there be another node waiting for a bast response ?

On 02/20/2014 07:18 AM, Jan Kara wrote:
> If we are dropping last inode reference from downconvert thread, we will
> end up calling ocfs2_mark_lockres_freeing() which can block if the lock
> we are freeing is queued thus creating an A-A deadlock. Luckily, since
> we are the downconvert thread, we can immediately dequeue the lock and
> thus avoid waiting in this case.
>
> Signed-off-by: Jan Kara <jack@suse.cz>
> ---
>   fs/ocfs2/dlmglue.c | 33 +++++++++++++++++++++++++++++++--
>   fs/ocfs2/dlmglue.h |  3 ++-
>   fs/ocfs2/inode.c   |  7 ++++---
>   3 files changed, 37 insertions(+), 6 deletions(-)
>
> diff --git a/fs/ocfs2/dlmglue.c b/fs/ocfs2/dlmglue.c
> index 19986959d149..b7580157ef01 100644
> --- a/fs/ocfs2/dlmglue.c
> +++ b/fs/ocfs2/dlmglue.c
> @@ -3150,7 +3150,8 @@ out:
>    * it safe to drop.
>    *
>    * You can *not* attempt to call cluster_lock on this lockres anymore. */
> -void ocfs2_mark_lockres_freeing(struct ocfs2_lock_res *lockres)
> +void ocfs2_mark_lockres_freeing(struct ocfs2_super *osb,
> +				struct ocfs2_lock_res *lockres)
>   {
>   	int status;
>   	struct ocfs2_mask_waiter mw;
> @@ -3160,6 +3161,33 @@ void ocfs2_mark_lockres_freeing(struct ocfs2_lock_res *lockres)
>   
>   	spin_lock_irqsave(&lockres->l_lock, flags);
>   	lockres->l_flags |= OCFS2_LOCK_FREEING;
> +	if (lockres->l_flags & OCFS2_LOCK_QUEUED && current == osb->dc_task) {
> +		unsigned long flags;
> +
> +		/*
> +		 * We know the downconvert is queued but not in progress
> +		 * because we are the downconvert thread and processing
> +		 * different lock. So we can just remove the lock from the
> +		 * queue. This is not only an optimization but also a way
> +		 * to avoid the following deadlock:
> +		 *   ocfs2_dentry_post_unlock()
> +		 *     ocfs2_dentry_lock_put()
> +		 *       ocfs2_drop_dentry_lock()
> +		 *         iput()
> +		 *           ocfs2_evict_inode()
> +		 *             ocfs2_clear_inode()
> +		 *               ocfs2_mark_lockres_freeing()
> +		 *                 ... blocks waiting for OCFS2_LOCK_QUEUED
> +		 *                 since we are the downconvert thread which
> +		 *                 should clear the flag.
> +		 */
> +		spin_lock_irqsave(&osb->dc_task_lock, flags);
> +		list_del_init(&lockres->l_blocked_list);
> +		osb->blocked_lock_count--;
> +		spin_unlock_irqrestore(&osb->dc_task_lock, flags);
> +		lockres_clear_flags(lockres, OCFS2_LOCK_QUEUED);
> +		goto out_unlock;
> +	}
>   	while (lockres->l_flags & OCFS2_LOCK_QUEUED) {
>   		lockres_add_mask_waiter(lockres, &mw, OCFS2_LOCK_QUEUED, 0);
>   		spin_unlock_irqrestore(&lockres->l_lock, flags);
> @@ -3172,6 +3200,7 @@ void ocfs2_mark_lockres_freeing(struct ocfs2_lock_res *lockres)
>   
>   		spin_lock_irqsave(&lockres->l_lock, flags);
>   	}
> +out_unlock:
>   	spin_unlock_irqrestore(&lockres->l_lock, flags);
>   }
>   
> @@ -3180,7 +3209,7 @@ void ocfs2_simple_drop_lockres(struct ocfs2_super *osb,
>   {
>   	int ret;
>   
> -	ocfs2_mark_lockres_freeing(lockres);
> +	ocfs2_mark_lockres_freeing(osb, lockres);
>   	ret = ocfs2_drop_lock(osb, lockres);
>   	if (ret)
>   		mlog_errno(ret);
> diff --git a/fs/ocfs2/dlmglue.h b/fs/ocfs2/dlmglue.h
> index 1d596d8c4a4a..d293a22c32c5 100644
> --- a/fs/ocfs2/dlmglue.h
> +++ b/fs/ocfs2/dlmglue.h
> @@ -157,7 +157,8 @@ int ocfs2_refcount_lock(struct ocfs2_refcount_tree *ref_tree, int ex);
>   void ocfs2_refcount_unlock(struct ocfs2_refcount_tree *ref_tree, int ex);
>   
>   
> -void ocfs2_mark_lockres_freeing(struct ocfs2_lock_res *lockres);
> +void ocfs2_mark_lockres_freeing(struct ocfs2_super *osb,
> +				struct ocfs2_lock_res *lockres);
>   void ocfs2_simple_drop_lockres(struct ocfs2_super *osb,
>   			       struct ocfs2_lock_res *lockres);
>   
> diff --git a/fs/ocfs2/inode.c b/fs/ocfs2/inode.c
> index 3b0d722de35e..9661f8db21dc 100644
> --- a/fs/ocfs2/inode.c
> +++ b/fs/ocfs2/inode.c
> @@ -1053,6 +1053,7 @@ static void ocfs2_clear_inode(struct inode *inode)
>   {
>   	int status;
>   	struct ocfs2_inode_info *oi = OCFS2_I(inode);
> +	struct ocfs2_super *osb = OCFS2_SB(inode->i_sb);
>   
>   	clear_inode(inode);
>   	trace_ocfs2_clear_inode((unsigned long long)oi->ip_blkno,
> @@ -1069,9 +1070,9 @@ static void ocfs2_clear_inode(struct inode *inode)
>   
>   	/* Do these before all the other work so that we don't bounce
>   	 * the downconvert thread while waiting to destroy the locks. */
> -	ocfs2_mark_lockres_freeing(&oi->ip_rw_lockres);
> -	ocfs2_mark_lockres_freeing(&oi->ip_inode_lockres);
> -	ocfs2_mark_lockres_freeing(&oi->ip_open_lockres);
> +	ocfs2_mark_lockres_freeing(osb, &oi->ip_rw_lockres);
> +	ocfs2_mark_lockres_freeing(osb, &oi->ip_inode_lockres);
> +	ocfs2_mark_lockres_freeing(osb, &oi->ip_open_lockres);
>   
>   	ocfs2_resv_discard(&OCFS2_SB(inode->i_sb)->osb_la_resmap,
>   			   &oi->ip_la_data_resv);