linux-kernel - Re: [PATCH v2] fs/file.c: don't acquire files->file_lock in fd

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Date:	Mon, 22 Jun 2015 03:32:07 +0100
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Mateusz Guzik <mguzik@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Yann Droneaud <ydroneaud@...eya.com>,
	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs/file.c: don't acquire files->file_lock in
 fd_install()

On Tue, Apr 28, 2015 at 09:25:03PM -0700, Eric Dumazet wrote:

> @@ -553,11 +572,20 @@ void __fd_install(struct files_struct *files, unsigned int fd,
>  		struct file *file)
>  {
>  	struct fdtable *fdt;
> -	spin_lock(&files->file_lock);
> -	fdt = files_fdtable(files);
> +
> +	rcu_read_lock_sched();
> +
> +	while (unlikely(files->resize_in_progress)) {
> +		rcu_read_unlock_sched();
> +		wait_event(files->resize_wait, !files->resize_in_progress);
> +		rcu_read_lock_sched();
> +	}
> +	/* coupled with smp_wmb() in expand_fdtable() */
> +	smp_rmb();
> +	fdt = rcu_dereference_sched(files->fdt);
>  	BUG_ON(fdt->fd[fd] != NULL);
>  	rcu_assign_pointer(fdt->fd[fd], file);
> -	spin_unlock(&files->file_lock);
> +	rcu_read_unlock_sched();

Umm...  You've taken something that was safe to use in atomic contexts
and turned into something that might wait for GFP_KERNEL allocation; what's
to guarantee that no users get broken by that?  At the very least, you want
to slap might_sleep() in there - the actual sleep is going to be very rare,
so it would be an extremely hard to reproduce and debug.

AFAICS, all current in-tree users should be safe, but fd_install() is exported
and quiet changes of that sort are rather antisocial.  Generally I don't give
a damn about out-of-tree code, but this one is over the top.

I _think_ it's otherwise OK, but please, add might_sleep() *AND* a note in
Documentation/filesystems/porting.

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