[PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

[PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

[Nikolay Borisov]

__del_reloc_root is called from:

a) Transaction commit via:
btrfs_transaction_commit
 commit_fs_roots
  btrfs_update_reloc_root
   __del_reloc_root

b) From the relocation thread with the following call chains:

relocate_block_group
 merge_reloc_roots
  insert_dirty_subvol
   btrfs_update_reloc_root
    __del_reloc_root

c) merge_reloc_roots
    free_reloc_roots
     __del_reloc_roots

(The above call chain can called from btrfs_recover_relocation as well
but for the purpose of this fix this is irrelevant).

The commont data structure that needs protecting is
fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list.
Turns out it's no needed to hold the trans_lock in __del_reloc_root
since consistency is already guaranteed by call chain b) above holding
a transaction while calling insert_dirty_subvol, meaning we cannot have
a concurrent transaction commit. For call chain c) above a snapshot of
the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and
free_reloc_roots is called on this local snapshot.

Those invariants are sufficient to prevent racing calls to
__del_reloc_root alongside other users of the list, as such it's fine
to drop the lock acquisition.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/relocation.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 8076c340749f..e5cb64409f7c 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root)
 		BUG_ON((struct btrfs_root *)node->data != root);
 	}
 
-	spin_lock(&fs_info->trans_lock);
 	list_del_init(&root->root_list);
-	spin_unlock(&fs_info->trans_lock);
 	kfree(node);
 }
 
-- 
2.17.1

[PATCH 2/2] btrfs: Use list_for_each_entry_safe in free_reloc_roots

[Nikolay Borisov]

The function always works on a local copy of the reloc root list, which
cannot be modified outside of it so using list_for_each_entry is fine.
Additionally the macro handles empty lists so drop list_empty checks of
callers. No semantic changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
---
 fs/btrfs/relocation.c | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index e5cb64409f7c..f13b79adf6b0 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2523,13 +2523,10 @@ int prepare_to_merge(struct reloc_control *rc, int err)
 static noinline_for_stack
 void free_reloc_roots(struct list_head *list)
 {
-	struct btrfs_root *reloc_root;
+	struct btrfs_root *reloc_root, *tmp;
 
-	while (!list_empty(list)) {
-		reloc_root = list_entry(list->next, struct btrfs_root,
-					root_list);
+	list_for_each_entry_safe(reloc_root, tmp, list, root_list)
 		__del_reloc_root(reloc_root);
-	}
 }
 
 static noinline_for_stack
@@ -2588,15 +2585,13 @@ void merge_reloc_roots(struct reloc_control *rc)
 out:
 	if (ret) {
 		btrfs_handle_fs_error(fs_info, ret, NULL);
-		if (!list_empty(&reloc_roots))
-			free_reloc_roots(&reloc_roots);
+		free_reloc_roots(&reloc_roots);
 
 		/* new reloc root may be added */
 		mutex_lock(&fs_info->reloc_mutex);
 		list_splice_init(&rc->reloc_roots, &reloc_roots);
 		mutex_unlock(&fs_info->reloc_mutex);
-		if (!list_empty(&reloc_roots))
-			free_reloc_roots(&reloc_roots);
+		free_reloc_roots(&reloc_roots);
 	}
 
 	BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
@@ -4689,8 +4684,7 @@ int btrfs_recover_relocation(struct btrfs_root *root)
 out_free:
 	kfree(rc);
 out:
-	if (!list_empty(&reloc_roots))
-		free_reloc_roots(&reloc_roots);
+	free_reloc_roots(&reloc_roots);
 
 	btrfs_free_path(path);
 
-- 
2.17.1

[PATCH v2 2/2] btrfs: Use list_for_each_entry_safe in free_reloc_roots

[Nikolay Borisov]

The function always works on a local copy of the reloc root list, which
cannot be modified outside of it so using list_for_each_entry is fine.
Additionally the macro handles empty lists so drop list_empty checks of
callers. No semantic changes.
---

V2: Base the patch on current misc-next rather than on a branch with Josef's
subvol cleanups part 2. 

 fs/btrfs/relocation.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
index 4b8c80de925b..a8898894f443 100644
--- a/fs/btrfs/relocation.c
+++ b/fs/btrfs/relocation.c
@@ -2523,11 +2523,9 @@ int prepare_to_merge(struct reloc_control *rc, int err)
 static noinline_for_stack
 void free_reloc_roots(struct list_head *list)
 {
-	struct btrfs_root *reloc_root;
+	struct btrfs_root *reloc_root, *tmp;
 
-	while (!list_empty(list)) {
-		reloc_root = list_entry(list->next, struct btrfs_root,
-					root_list);
+	list_for_each_entry_safe(reloc_root, tmp, list, root_list) {
 		__del_reloc_root(reloc_root);
 		free_extent_buffer(reloc_root->node);
 		free_extent_buffer(reloc_root->commit_root);
@@ -2592,15 +2590,13 @@ void merge_reloc_roots(struct reloc_control *rc)
 out:
 	if (ret) {
 		btrfs_handle_fs_error(fs_info, ret, NULL);
-		if (!list_empty(&reloc_roots))
-			free_reloc_roots(&reloc_roots);
+		free_reloc_roots(&reloc_roots);
 
 		/* new reloc root may be added */
 		mutex_lock(&fs_info->reloc_mutex);
 		list_splice_init(&rc->reloc_roots, &reloc_roots);
 		mutex_unlock(&fs_info->reloc_mutex);
-		if (!list_empty(&reloc_roots))
-			free_reloc_roots(&reloc_roots);
+		free_reloc_roots(&reloc_roots);
 	}
 
 	BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
@@ -4693,8 +4689,7 @@ int btrfs_recover_relocation(struct btrfs_root *root)
 out_free:
 	kfree(rc);
 out:
-	if (!list_empty(&reloc_roots))
-		free_reloc_roots(&reloc_roots);
+	free_reloc_roots(&reloc_roots);
 
 	btrfs_free_path(path);
 
-- 
2.17.1

Re: [PATCH 2/2] btrfs: Use list_for_each_entry_safe in free_reloc_roots

[Qu Wenruo]

On 2020/2/21 下午9:11, Nikolay Borisov wrote:
> The function always works on a local copy of the reloc root list, which
> cannot be modified outside of it so using list_for_each_entry is fine.
> Additionally the macro handles empty lists so drop list_empty checks of
> callers. No semantic changes.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu
> ---
>  fs/btrfs/relocation.c | 16 +++++-----------
>  1 file changed, 5 insertions(+), 11 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index e5cb64409f7c..f13b79adf6b0 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -2523,13 +2523,10 @@ int prepare_to_merge(struct reloc_control *rc, int err)
>  static noinline_for_stack
>  void free_reloc_roots(struct list_head *list)
>  {
> -	struct btrfs_root *reloc_root;
> +	struct btrfs_root *reloc_root, *tmp;
>
> -	while (!list_empty(list)) {
> -		reloc_root = list_entry(list->next, struct btrfs_root,
> -					root_list);
> +	list_for_each_entry_safe(reloc_root, tmp, list, root_list)
>  		__del_reloc_root(reloc_root);
> -	}
>  }
>
>  static noinline_for_stack
> @@ -2588,15 +2585,13 @@ void merge_reloc_roots(struct reloc_control *rc)
>  out:
>  	if (ret) {
>  		btrfs_handle_fs_error(fs_info, ret, NULL);
> -		if (!list_empty(&reloc_roots))
> -			free_reloc_roots(&reloc_roots);
> +		free_reloc_roots(&reloc_roots);
>
>  		/* new reloc root may be added */
>  		mutex_lock(&fs_info->reloc_mutex);
>  		list_splice_init(&rc->reloc_roots, &reloc_roots);
>  		mutex_unlock(&fs_info->reloc_mutex);
> -		if (!list_empty(&reloc_roots))
> -			free_reloc_roots(&reloc_roots);
> +		free_reloc_roots(&reloc_roots);
>  	}
>
>  	BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
> @@ -4689,8 +4684,7 @@ int btrfs_recover_relocation(struct btrfs_root *root)
>  out_free:
>  	kfree(rc);
>  out:
> -	if (!list_empty(&reloc_roots))
> -		free_reloc_roots(&reloc_roots);
> +	free_reloc_roots(&reloc_roots);
>
>  	btrfs_free_path(path);
>
>

Re: [PATCH 2/2] btrfs: Use list_for_each_entry_safe in free_reloc_roots

[David Sterba]

On Fri, Feb 21, 2020 at 03:11:24PM +0200, Nikolay Borisov wrote:
> The function always works on a local copy of the reloc root list, which
> cannot be modified outside of it so using list_for_each_entry is fine.
> Additionally the macro handles empty lists so drop list_empty checks of
> callers. No semantic changes.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

This is still relevant for misc-next, so I'm applying it to misc-next.

Re: [PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

[Qu Wenruo]

On 2020/2/21 下午9:11, Nikolay Borisov wrote:
> __del_reloc_root is called from:
>
> a) Transaction commit via:
> btrfs_transaction_commit
>  commit_fs_roots
>   btrfs_update_reloc_root
>    __del_reloc_root
>
> b) From the relocation thread with the following call chains:
>
> relocate_block_group
>  merge_reloc_roots
>   insert_dirty_subvol
>    btrfs_update_reloc_root
>     __del_reloc_root
>
> c) merge_reloc_roots
>     free_reloc_roots
>      __del_reloc_roots
>
> (The above call chain can called from btrfs_recover_relocation as well
> but for the purpose of this fix this is irrelevant).
>
> The commont data structure that needs protecting is
> fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list.
> Turns out it's no needed to hold the trans_lock in __del_reloc_root
> since consistency is already guaranteed by call chain b) above holding
> a transaction while calling insert_dirty_subvol, meaning we cannot have
> a concurrent transaction commit. For call chain c) above a snapshot of
> the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and
> free_reloc_roots is called on this local snapshot.
>
> Those invariants are sufficient to prevent racing calls to
> __del_reloc_root alongside other users of the list, as such it's fine
> to drop the lock acquisition.
>
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Reviewed-by: Qu Wenruo <wqu@suse.com>

Thanks,
Qu

> ---
>  fs/btrfs/relocation.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 8076c340749f..e5cb64409f7c 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root)
>  		BUG_ON((struct btrfs_root *)node->data != root);
>  	}
>
> -	spin_lock(&fs_info->trans_lock);
>  	list_del_init(&root->root_list);
> -	spin_unlock(&fs_info->trans_lock);
>  	kfree(node);
>  }
>
>

Re: [PATCH 1/2] btrfs: Remove superflous lock acquisition in __del_reloc_root

[David Sterba]

On Fri, Feb 21, 2020 at 03:11:23PM +0200, Nikolay Borisov wrote:
> __del_reloc_root is called from:
> 
> a) Transaction commit via:
> btrfs_transaction_commit
>  commit_fs_roots
>   btrfs_update_reloc_root
>    __del_reloc_root
> 
> b) From the relocation thread with the following call chains:
> 
> relocate_block_group
>  merge_reloc_roots
>   insert_dirty_subvol
>    btrfs_update_reloc_root
>     __del_reloc_root
> 
> c) merge_reloc_roots
>     free_reloc_roots
>      __del_reloc_roots
> 
> (The above call chain can called from btrfs_recover_relocation as well
> but for the purpose of this fix this is irrelevant).
> 
> The commont data structure that needs protecting is
> fs_info->reloc_ctl->reloc_list as reloc roots are anchored at this list.
> Turns out it's no needed to hold the trans_lock in __del_reloc_root
> since consistency is already guaranteed by call chain b) above holding
> a transaction while calling insert_dirty_subvol, meaning we cannot have
> a concurrent transaction commit. For call chain c) above a snapshot of
> the fs_info->reloc_ctl->reloc_list is taken with reloc_mutex held and
> free_reloc_roots is called on this local snapshot.
> 
> Those invariants are sufficient to prevent racing calls to
> __del_reloc_root alongside other users of the list, as such it's fine
> to drop the lock acquisition.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> ---
>  fs/btrfs/relocation.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/fs/btrfs/relocation.c b/fs/btrfs/relocation.c
> index 8076c340749f..e5cb64409f7c 100644
> --- a/fs/btrfs/relocation.c
> +++ b/fs/btrfs/relocation.c
> @@ -1381,9 +1381,7 @@ static void __del_reloc_root(struct btrfs_root *root)
>  		BUG_ON((struct btrfs_root *)node->data != root);
>  	}
>  
> -	spin_lock(&fs_info->trans_lock);
>  	list_del_init(&root->root_list);
> -	spin_unlock(&fs_info->trans_lock);

This has been obsoleted by patch f44deb7442edf42abee6 ("btrfs: hold a
ref on the root->reloc_root"), I think the locks are still needed but
you may want take a look.