Re: [PATCH 04/11] fs: Use acquire/release for fdtable resize synchronization

[Jan Kara]

On Tue 25-08-26 17:54:15, Jinjie Ruan wrote:
> Replace the smp_wmb()/smp_rmb() barrier pair with
> smp_store_release()/smp_load_acquire() on `files->resize_in_progress`.
> The flag is the publish point for fdtable expansion: writers clear it
> via release after rcu_assign_pointer(), readers check it via acquire
> before rcu_dereference_sched(). Observing it clear guarantees the new
> fdt pointer is visible.
> 
> This expresses the pattern more clearly and allows cheaper one-way
> barriers on weakly-ordered architectures (e.g. arm64 STLR/LDAR vs
> DMB ISHST/ISHLD).
> 
> Assisted-by: DeepSeek:DeepSeek-V3
> Signed-off-by: Jinjie Ruan <ruanjinjie@xxxxxxxxxx>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@xxxxxxx>

								Honza

> ---
>  fs/file.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/fs/file.c b/fs/file.c
> index 628ca07dc4b1..86c035d459f2 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -273,8 +273,6 @@ static int expand_fdtable(struct files_struct *files, unsigned int nr)
>  	rcu_assign_pointer(files->fdt, new_fdt);
>  	if (cur_fdt != &files->fdtab)
>  		call_rcu(&cur_fdt->rcu, free_fdtable_rcu);
> -	/* coupled with smp_rmb() in fd_install() */
> -	smp_wmb();
>  	return 0;
>  }
>  
> @@ -313,7 +311,8 @@ static int expand_files(struct files_struct *files, unsigned int nr)
>  	/* All good, so we try */
>  	files->resize_in_progress = true;
>  	error = expand_fdtable(files, nr);
> -	files->resize_in_progress = false;
> +	/* coupled with smp_load_acquire() in fd_install() */
> +	smp_store_release(&files->resize_in_progress, false);
>  
>  	wake_up_all(&files->resize_wait);
>  	return error;
> @@ -685,13 +684,12 @@ void fd_install(unsigned int fd, struct file *file)
>  		return;
>  
>  	rcu_read_lock_sched();
> -	if (unlikely(files->resize_in_progress)) {
> +	/* coupled with smp_store_release() in expand_files() */
> +	if (unlikely(smp_load_acquire(&files->resize_in_progress))) {
>  		rcu_read_unlock_sched();
>  		fd_install_slowpath(fd, file);
>  		return;
>  	}
> -	/* coupled with smp_wmb() in expand_fdtable() */
> -	smp_rmb();
>  	fdt = rcu_dereference_sched(files->fdt);
>  	VFS_BUG_ON(rcu_access_pointer(fdt->fd[fd]) != NULL);
>  	rcu_assign_pointer(fdt->fd[fd], file);
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR