Re: [PATCH 10/22] fast_dput(): new rules for refcount

[Christian Brauner <brauner@kernel.org>]

On Thu, Nov 09, 2023 at 06:20:44AM +0000, Al Viro wrote:
> Currently the "need caller to do more work" path in fast_dput()
> has refcount decremented, then, with ->d_lock held and
> refcount verified to have reached 0 fast_dput() forcibly resets
> the refcount to 1.
> 
> Move that resetting refcount to 1 into the callers; later in
> the series it will be massaged out of existence.
> 
> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
> ---

Ok, this is safe to do because of

[PATCH 09/22] fast_dput(): handle underflows gracefully
https://lore.kernel.org/linux-fsdevel/20231109062056.3181775-9-viro@zeniv.linux.org.uk

as return false from fast_dput() now always means refcount is zero.

Reviewed-by: Christian Brauner <brauner@kernel.org>

>  fs/dcache.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/dcache.c b/fs/dcache.c
> index e02b3c81bc02..9a3eeee02500 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -847,13 +847,6 @@ static inline bool fast_dput(struct dentry *dentry)
>  		spin_unlock(&dentry->d_lock);
>  		return true;
>  	}
> -
> -	/*
> -	 * Re-get the reference we optimistically dropped. We hold the
> -	 * lock, and we just tested that it was zero, so we can just
> -	 * set it to 1.
> -	 */
> -	dentry->d_lockref.count = 1;
>  	return false;
>  }
>  
> @@ -896,6 +889,7 @@ void dput(struct dentry *dentry)
>  		}
>  
>  		/* Slow case: now with the dentry lock held */
> +		dentry->d_lockref.count = 1;
>  		rcu_read_unlock();
>  
>  		if (likely(retain_dentry(dentry))) {
> @@ -930,6 +924,7 @@ void dput_to_list(struct dentry *dentry, struct list_head *list)
>  		return;
>  	}
>  	rcu_read_unlock();
> +	dentry->d_lockref.count = 1;
>  	if (!retain_dentry(dentry))
>  		__dput_to_list(dentry, list);
>  	spin_unlock(&dentry->d_lock);
> -- 
> 2.39.2
>