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@xxxxxxxxx>
Looks good. Feel free to add:
Reviewed-by: Jan Kara <jack@xxxxxxx>
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@xxxxxxxx>
SUSE Labs, CR
