linux-kernel - Re: [PATCH 2/2] kernfs: remove mutex in kernfs_dop

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Date:   Wed, 2 Dec 2020 19:27:38 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Fox Chen <foxhlchen@...il.com>
Cc:     tj@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] kernfs: remove mutex in kernfs_dop_revalidate

On Wed, Dec 02, 2020 at 10:58:37PM +0800, Fox Chen wrote:
> There is a big mutex in kernfs_dop_revalidate which slows down the
> concurrent performance of kernfs.
> 
> Since kernfs_dop_revalidate only does some checks, the lock is
> largely unnecessary. Also, according to kernel filesystem locking
> document:
> https://www.kernel.org/doc/html/latest/filesystems/locking.html
> locking is not in the protocal for d_revalidate operation.
> 
> This patch remove this mutex from
> kernfs_dop_revalidate, so kernfs_dop_revalidate
> can run concurrently.
> 
> Signed-off-by: Fox Chen <foxhlchen@...il.com>
> ---
>  fs/kernfs/dir.c | 9 +++------
>  1 file changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
> index 9aec80b9d7c6..c2267c93f546 100644
> --- a/fs/kernfs/dir.c
> +++ b/fs/kernfs/dir.c
> @@ -26,7 +26,6 @@ static DEFINE_SPINLOCK(kernfs_idr_lock);	/* root->ino_idr */
>  
>  static bool kernfs_active(struct kernfs_node *kn)
>  {
> -	lockdep_assert_held(&kernfs_mutex);
>  	return atomic_read(&kn->active) >= 0;
>  }
>  
> @@ -557,10 +556,9 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
>  
>  	/* Always perform fresh lookup for negatives */
>  	if (d_really_is_negative(dentry))
> -		goto out_bad_unlocked;
> +		goto out_bad;
>  
>  	kn = kernfs_dentry_node(dentry);
> -	mutex_lock(&kernfs_mutex);
>  
>  	/* The kernfs node has been deactivated */
>  	if (!kernfs_active(kn))
> @@ -579,11 +577,8 @@ static int kernfs_dop_revalidate(struct dentry *dentry, unsigned int flags)
>  	    kernfs_info(dentry->d_sb)->ns != kn->ns)
>  		goto out_bad;
>  
> -	mutex_unlock(&kernfs_mutex);
>  	return 1;
>  out_bad:
> -	mutex_unlock(&kernfs_mutex);
> -out_bad_unlocked:
>  	return 0;
>  }
>  
> @@ -650,6 +645,8 @@ static struct kernfs_node *__kernfs_new_node(struct kernfs_root *root,
>  	kn->mode = mode;
>  	kn->flags = flags;
>  
> +	rwlock_init(&kn->iattr_rwlock);

Ah, now you initialize this, it should go into patch 1, right? :)

greg k-h