linux-kernel - Re: [PATCH v2] fs: reduce work in fdget

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Message-ID: <7trrmztdpoyqcbarrhir4kiayc7yrqfznfyrrt5f5flyfmgu6u@5sx3yapl7bcv>
Date: Thu, 20 Mar 2025 11:33:43 +0100
From: Jan Kara <jack@...e.cz>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: brauner@...nel.org, viro@...iv.linux.org.uk, jack@...e.cz, 
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] fs: reduce work in fdget_pos()

On Wed 19-03-25 22:58:01, Mateusz Guzik wrote:
> 1. predict the file was found
> 2. explicitly compare the ref to "one", ignoring the dead zone
> 
> The latter arguably improves the behavior to begin with. Suppose the
> count turned bad -- the previously used ref routine is going to check
> for it and return 0, indicating the count does not necessitate taking
> ->f_pos_lock. But there very well may be several users.
> 
> i.e. not paying for special-casing the dead zone improves semantics.
> 
> While here spell out each condition in a dedicated if statement. This
> has no effect on generated code.
> 
> Sizes are as follows (in bytes; gcc 13, x86-64):
> stock:		321
> likely(): 	298
> likely()+ref:	280
> 
> Signed-off-by: Mateusz Guzik <mjguzik@...il.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> diff --git a/fs/file.c b/fs/file.c
> index ddefb5c80398..0e919bed6058 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -1192,8 +1192,13 @@ struct fd fdget_raw(unsigned int fd)
>   */
>  static inline bool file_needs_f_pos_lock(struct file *file)
>  {
> -	return (file->f_mode & FMODE_ATOMIC_POS) &&
> -		(file_count(file) > 1 || file->f_op->iterate_shared);
> +	if (!(file->f_mode & FMODE_ATOMIC_POS))
> +		return false;
> +	if (__file_ref_read_raw(&file->f_ref) != FILE_REF_ONEREF)
> +		return true;
> +	if (file->f_op->iterate_shared)
> +		return true;
> +	return false;
>  }
>  
>  bool file_seek_cur_needs_f_lock(struct file *file)
> @@ -1211,7 +1216,7 @@ struct fd fdget_pos(unsigned int fd)
>  	struct fd f = fdget(fd);
>  	struct file *file = fd_file(f);
>  
> -	if (file && file_needs_f_pos_lock(file)) {
> +	if (likely(file) && file_needs_f_pos_lock(file)) {
>  		f.word |= FDPUT_POS_UNLOCK;
>  		mutex_lock(&file->f_pos_lock);
>  	}
> diff --git a/include/linux/file_ref.h b/include/linux/file_ref.h
> index 6ef92d765a66..7db62fbc0500 100644
> --- a/include/linux/file_ref.h
> +++ b/include/linux/file_ref.h
> @@ -208,4 +208,18 @@ static inline unsigned long file_ref_read(file_ref_t *ref)
>  	return c >= FILE_REF_RELEASED ? 0 : c + 1;
>  }
>  
> +/*
> + * __file_ref_read_raw - Return the value stored in ref->refcnt
> + * @ref: Pointer to the reference count
> + *
> + * Return: The raw value found in the counter
> + *
> + * A hack for file_needs_f_pos_lock(), you probably want to use
> + * file_ref_read() instead.
> + */
> +static inline unsigned long __file_ref_read_raw(file_ref_t *ref)
> +{
> +	return atomic_long_read(&ref->refcnt);
> +}
> +
>  #endif
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR