linux-kernel - Re: [PATCH v2 1/6] super: remove pointless s

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Message-ID: <20250611162629.GE6138@frogsfrogsfrogs>
Date: Wed, 11 Jun 2025 09:26:29 -0700
From: "Darrick J. Wong" <djwong@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: linux-fsdevel@...r.kernel.org, jack@...e.cz,
	linux-kernel@...r.kernel.org,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	mcgrof@...nel.org, hch@...radead.org, david@...morbit.com,
	rafael@...nel.org, pavel@...nel.org, peterz@...radead.org,
	mingo@...hat.com, will@...nel.org, boqun.feng@...il.com
Subject: Re: [PATCH v2 1/6] super: remove pointless s_root checks

On Sat, Mar 29, 2025 at 09:42:14AM +0100, Christian Brauner wrote:
> The locking guarantees that the superblock is alive and sb->s_root is
> still set. Remove the pointless check.
> 
> Signed-off-by: Christian Brauner <brauner@...nel.org>
> ---
>  fs/super.c | 19 ++++++-------------
>  1 file changed, 6 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index 97a17f9d9023..dc14f4bf73a6 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -930,8 +930,7 @@ void iterate_supers(void (*f)(struct super_block *, void *), void *arg)
>  
>  		locked = super_lock_shared(sb);
>  		if (locked) {
> -			if (sb->s_root)
> -				f(sb, arg);
> +			f(sb, arg);
>  			super_unlock_shared(sb);
>  		}
>  
> @@ -967,11 +966,8 @@ void iterate_supers_type(struct file_system_type *type,
>  		spin_unlock(&sb_lock);
>  
>  		locked = super_lock_shared(sb);
> -		if (locked) {
> -			if (sb->s_root)
> -				f(sb, arg);
> -			super_unlock_shared(sb);
> -		}
> +		if (locked)
> +			f(sb, arg);

Hey Christian,

I might be trying to be the second(?) user of iterate_supers_type[1]. :)

This change removes the call to super_unlock_shared, which means that
iterate_supers_type returns with the super_lock(s) still held.  I'm
guessing that this is a bug and not an intentional change to require the
callback to call super_unlock_shared, right?

--D

[1] https://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux.git/commit/?h=health-monitoring&id=3ae9b1d43dcdeaa38e93dc400d1871872ba0e27f

>  
>  		spin_lock(&sb_lock);
>  		if (p)
> @@ -991,18 +987,15 @@ struct super_block *user_get_super(dev_t dev, bool excl)
>  
>  	spin_lock(&sb_lock);
>  	list_for_each_entry(sb, &super_blocks, s_list) {
> -		if (sb->s_dev ==  dev) {
> +		if (sb->s_dev == dev) {
>  			bool locked;
>  
>  			sb->s_count++;
>  			spin_unlock(&sb_lock);
>  			/* still alive? */
>  			locked = super_lock(sb, excl);
> -			if (locked) {
> -				if (sb->s_root)
> -					return sb;
> -				super_unlock(sb, excl);
> -			}
> +			if (locked)
> +				return sb; /* caller will drop */
>  			/* nope, got unmounted */
>  			spin_lock(&sb_lock);
>  			__put_super(sb);
> 
> -- 
> 2.47.2
> 
>