Re: [PATCH v3 0/3] fsnotify: fix softlockups iterating over d_subdirs

[Stephen Brennan]

Amir Goldstein <amir73il@xxxxxxxxx> writes:
[...]
> IIUC, patches #1+#3 fix a reproducible softlock, so if Al approves them,
> I see no reason to withhold.
>
> With patches #1+#3 applied, the penalty is restricted to adding or
> removing/destroying multiple marks on very large dirs or dirs with
> many negative dentries.
>
> I think that fixing the synthetic test case of multiple added marks
> is rather easy even without DCACHE_FSNOTIFY_PARENT_WATCHED.
> Something like the attached patch is what Jan had suggested in the
> first place.
>
> The synthetic test case of concurrent add/remove watch on the same
> dir may still result in multiple children iterations, but that will usually
> not be avoided even with DCACHE_FSNOTIFY_PARENT_WATCHED
> and probably not worth optimizing for.
>
> Thoughts?

If I understand your train of thought, your below patch would take the
place of my patch #2, since #3 requires we not hold a spinlock during
fsnotify_update_children_dentry_flags().

And since you fully rely on dentry accessors to lazily clear flags, you
don't need to have the mutual exclusion provided by the inode lock.

I like that a lot.

However, the one thing I see here is that if we no longer hold the inode
lock, patch #3 needs to be re-examined: it assumes that dentries can't
be removed or moved around, and that assumption is only true with the
inode lock held.

I'll test things out with this patch replacing #2 and see how things
shake out. Maybe I can improve #3.

Thanks,
Stephen

>
> Thanks,
> Amir.
> From c8ea1d84397c26ce334bff9d9f721400cebb88dd Mon Sep 17 00:00:00 2001
> From: Amir Goldstein <amir73il@xxxxxxxxx>
> Date: Wed, 2 Nov 2022 10:28:01 +0200
> Subject: [PATCH] fsnotify: clear PARENT_WATCHED flags lazily
>
> Call fsnotify_update_children_dentry_flags() to set PARENT_WATCHED flags
> only when parent starts watching children.
>
> When parent stops watching children, clear false positive PARENT_WATCHED
> flags lazily in __fsnotify_parent() for each accessed child.
>
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
> ---
>  fs/notify/fsnotify.c             | 26 ++++++++++++++++++++------
>  fs/notify/fsnotify.h             |  3 ++-
>  fs/notify/mark.c                 | 32 +++++++++++++++++++++++++++++---
>  include/linux/fsnotify_backend.h |  8 +++++---
>  4 files changed, 56 insertions(+), 13 deletions(-)
>
> diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
> index 1e541a9bd12b..f60078d6bb97 100644
> --- a/fs/notify/fsnotify.c
> +++ b/fs/notify/fsnotify.c
> @@ -103,17 +103,13 @@ void fsnotify_sb_delete(struct super_block *sb)
>   * parent cares.  Thus when an event happens on a child it can quickly tell
>   * if there is a need to find a parent and send the event to the parent.
>   */
> -void __fsnotify_update_child_dentry_flags(struct inode *inode)
> +void fsnotify_update_children_dentry_flags(struct inode *inode, bool watched)
>  {
>  	struct dentry *alias;
> -	int watched;
>  
>  	if (!S_ISDIR(inode->i_mode))
>  		return;
>  
> -	/* determine if the children should tell inode about their events */
> -	watched = fsnotify_inode_watches_children(inode);
> -
>  	spin_lock(&inode->i_lock);
>  	/* run all of the dentries associated with this inode.  Since this is a
>  	 * directory, there damn well better only be one item on this list */
> @@ -140,6 +136,24 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
>  	spin_unlock(&inode->i_lock);
>  }
>  
> +/*
> + * Lazily clear false positive PARENT_WATCHED flag for child whose parent had
> + * stopped wacthing children.
> + */
> +static void fsnotify_update_child_dentry_flags(struct inode *inode,
> +					       struct dentry *dentry)
> +{
> +	spin_lock(&dentry->d_lock);
> +	/*
> +	 * d_lock is a sufficient barrier to prevent observing a non-watched
> +	 * parent state from before the fsnotify_update_children_dentry_flags()
> +	 * or fsnotify_update_flags() call that had set PARENT_WATCHED.
> +	 */
> +	if (!fsnotify_inode_watches_children(inode))
> +		dentry->d_flags &= ~DCACHE_FSNOTIFY_PARENT_WATCHED;
> +	spin_unlock(&dentry->d_lock);
> +}
> +
>  /* Are inode/sb/mount interested in parent and name info with this event? */
>  static bool fsnotify_event_needs_parent(struct inode *inode, struct mount *mnt,
>  					__u32 mask)
> @@ -208,7 +222,7 @@ int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
>  	p_inode = parent->d_inode;
>  	p_mask = fsnotify_inode_watches_children(p_inode);
>  	if (unlikely(parent_watched && !p_mask))
> -		__fsnotify_update_child_dentry_flags(p_inode);
> +		fsnotify_update_child_dentry_flags(p_inode, dentry);
>  
>  	/*
>  	 * Include parent/name in notification either if some notification
> diff --git a/fs/notify/fsnotify.h b/fs/notify/fsnotify.h
> index fde74eb333cc..bce9be36d06b 100644
> --- a/fs/notify/fsnotify.h
> +++ b/fs/notify/fsnotify.h
> @@ -74,7 +74,8 @@ static inline void fsnotify_clear_marks_by_sb(struct super_block *sb)
>   * update the dentry->d_flags of all of inode's children to indicate if inode cares
>   * about events that happen to its children.
>   */
> -extern void __fsnotify_update_child_dentry_flags(struct inode *inode);
> +extern void fsnotify_update_children_dentry_flags(struct inode *inode,
> +						  bool watched);
>  
>  extern struct kmem_cache *fsnotify_mark_connector_cachep;
>  
> diff --git a/fs/notify/mark.c b/fs/notify/mark.c
> index fcc68b8a40fd..614bce0e7261 100644
> --- a/fs/notify/mark.c
> +++ b/fs/notify/mark.c
> @@ -176,6 +176,24 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>  	return fsnotify_update_iref(conn, want_iref);
>  }
>  
> +static bool fsnotify_conn_watches_children(
> +					struct fsnotify_mark_connector *conn)
> +{
> +	if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
> +		return false;
> +
> +	return fsnotify_inode_watches_children(fsnotify_conn_inode(conn));
> +}
> +
> +static void fsnotify_conn_set_children_dentry_flags(
> +					struct fsnotify_mark_connector *conn)
> +{
> +	if (conn->type != FSNOTIFY_OBJ_TYPE_INODE)
> +		return;
> +
> +	fsnotify_update_children_dentry_flags(fsnotify_conn_inode(conn), true);
> +}
> +
>  /*
>   * Calculate mask of events for a list of marks. The caller must make sure
>   * connector and connector->obj cannot disappear under us.  Callers achieve
> @@ -184,15 +202,23 @@ static void *__fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>   */
>  void fsnotify_recalc_mask(struct fsnotify_mark_connector *conn)
>  {
> +	bool update_children;
> +
>  	if (!conn)
>  		return;
>  
>  	spin_lock(&conn->lock);
> +	update_children = !fsnotify_conn_watches_children(conn);
>  	__fsnotify_recalc_mask(conn);
> +	update_children &= fsnotify_conn_watches_children(conn);
>  	spin_unlock(&conn->lock);
> -	if (conn->type == FSNOTIFY_OBJ_TYPE_INODE)
> -		__fsnotify_update_child_dentry_flags(
> -					fsnotify_conn_inode(conn));
> +	/*
> +	 * Set children's PARENT_WATCHED flags only if parent started watching.
> +	 * When parent stops watching, we clear false positive PARENT_WATCHED
> +	 * flags lazily in __fsnotify_parent().
> +	 */
> +	if (update_children)
> +		fsnotify_conn_set_children_dentry_flags(conn);
>  }
>  
>  /* Free all connectors queued for freeing once SRCU period ends */
> diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> index a31423c376a7..bd90bcf6c3b0 100644
> --- a/include/linux/fsnotify_backend.h
> +++ b/include/linux/fsnotify_backend.h
> @@ -586,12 +586,14 @@ static inline __u32 fsnotify_parent_needed_mask(__u32 mask)
>  
>  static inline int fsnotify_inode_watches_children(struct inode *inode)
>  {
> +	__u32 parent_mask = READ_ONCE(inode->i_fsnotify_mask);
> +
>  	/* FS_EVENT_ON_CHILD is set if the inode may care */
> -	if (!(inode->i_fsnotify_mask & FS_EVENT_ON_CHILD))
> +	if (!(parent_mask & FS_EVENT_ON_CHILD))
>  		return 0;
>  	/* this inode might care about child events, does it care about the
>  	 * specific set of events that can happen on a child? */
> -	return inode->i_fsnotify_mask & FS_EVENTS_POSS_ON_CHILD;
> +	return parent_mask & FS_EVENTS_POSS_ON_CHILD;
>  }
>  
>  /*
> @@ -605,7 +607,7 @@ static inline void fsnotify_update_flags(struct dentry *dentry)
>  	/*
>  	 * Serialisation of setting PARENT_WATCHED on the dentries is provided
>  	 * by d_lock. If inotify_inode_watched changes after we have taken
> -	 * d_lock, the following __fsnotify_update_child_dentry_flags call will
> +	 * d_lock, the following fsnotify_update_children_dentry_flags call will
>  	 * find our entry, so it will spin until we complete here, and update
>  	 * us with the new state.
>  	 */
> -- 
> 2.25.1