linux-kernel - Re: [RFC PATCH] fs: elide the smp_rmb fence in fd

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Message-ID: <20241205141850.GS3387508@ZenIV>
Date: Thu, 5 Dec 2024 14:18:50 +0000
From: Al Viro <viro@...iv.linux.org.uk>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: paulmck@...nel.org, brauner@...nel.org, jack@...e.cz,
	linux-fsdevel@...r.kernel.org, torvalds@...ux-foundation.org,
	edumazet@...gle.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] fs: elide the smp_rmb fence in fd_install()

On Thu, Dec 05, 2024 at 01:03:32PM +0100, Mateusz Guzik wrote:
>  void fd_install(unsigned int fd, struct file *file)
>  {
> -	struct files_struct *files = current->files;
> +	struct files_struct *files;
>  	struct fdtable *fdt;
>  
>  	if (WARN_ON_ONCE(unlikely(file->f_mode & FMODE_BACKING)))
>  		return;
>  
> +	/*
> +	 * Synchronized with expand_fdtable(), see that routine for an
> +	 * explanation.
> +	 */
>  	rcu_read_lock_sched();
> +	files = READ_ONCE(current->files);

What are you trying to do with that READ_ONCE()?  current->files
itself is *not* changed by any of that code; current->files->fdtab is.