linux-kernel - Re: [RFC PATCH] file: Wrap locking mechanism for f_pos

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Message-ID: <20241204102644.hvutdftkueiiyss7@quack3>
Date: Wed, 4 Dec 2024 11:26:44 +0100
From: Jan Kara <jack@...e.cz>
To: I Hsin Cheng <richard120310@...il.com>
Cc: viro@...iv.linux.org.uk, brauner@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] file: Wrap locking mechanism for f_pos_lock

On Wed 04-12-24 17:23:25, I Hsin Cheng wrote:
> As the implementation of "f->f_pos_lock" may change in the future,
> wrapping the actual implementation of locking and unlocking of it can
> provide better decoupling semantics.
> 
> "__f_unlock_pos()" already exist and does that, adding "__f_lock_pos()"
> can provide full decoupling.
> 
> Signed-off-by: I Hsin Cheng <richard120310@...il.com>

I guess this would make sense for consistence. But Al, what was the
motivation of introducing __f_unlock_pos() in the first place? It has one
caller and was silently introduced in 63b6df14134d ("give
readdir(2)/getdents(2)/etc. uniform exclusion with lseek()") about 8 years
ago.

								Honza


> ---
>  fs/file.c            | 7 ++++++-
>  include/linux/file.h | 1 +
>  2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index fb1011cf6b4a..b93ac67d276d 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1181,6 +1181,11 @@ static inline bool file_needs_f_pos_lock(struct file *file)
>  		(file_count(file) > 1 || file->f_op->iterate_shared);
>  }
>  
> +void __f_lock_pos(struct file *f)
> +{
> +	mutex_lock(&f->f_pos_lock);
> +}
> +
>  struct fd fdget_pos(unsigned int fd)
>  {
>  	struct fd f = fdget(fd);
> @@ -1188,7 +1193,7 @@ struct fd fdget_pos(unsigned int fd)
>  
>  	if (file && file_needs_f_pos_lock(file)) {
>  		f.word |= FDPUT_POS_UNLOCK;
> -		mutex_lock(&file->f_pos_lock);
> +		__f_lock_pos(file);
>  	}
>  	return f;
>  }
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 302f11355b10..16292bd95499 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -67,6 +67,7 @@ extern struct file *fget(unsigned int fd);
>  extern struct file *fget_raw(unsigned int fd);
>  extern struct file *fget_task(struct task_struct *task, unsigned int fd);
>  extern struct file *fget_task_next(struct task_struct *task, unsigned int *fd);
> +extern void __f_lock_pos(struct file *file);
>  extern void __f_unlock_pos(struct file *);
>  
>  struct fd fdget(unsigned int fd);
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR