linux-kernel - Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

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Message-ID: <20120215190622.GA25559@redhat.com>
Date:	Wed, 15 Feb 2012 20:06:22 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrey Vagin <avagin@...nvz.org>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Glauber Costa <glommer@...allels.com>,
	Andi Kleen <andi@...stfloor.org>, Tejun Heo <tj@...nel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Pekka Enberg <penberg@...nel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Alexey Dobriyan <adobriyan@...il.com>, Valdis.Kletnieks@...edu,
	Michal Marek <mmarek@...e.cz>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: + syscalls-x86-add-__nr_kcmp-syscall-v8.patch added to -mm tree

On 02/15, Cyrill Gorcunov wrote:
>
> On Wed, Feb 15, 2012 at 07:10:08PM +0400, Cyrill Gorcunov wrote:
> > On Wed, Feb 15, 2012 at 03:36:06PM +0100, Oleg Nesterov wrote:
> > >
> > > task->files can be NULL, we can race with exit_files().
> >
> > So I need to call get_files_struct. Thanks Oleg!
> >
>
> Something like below I think, right? (it seems this
> patc doesn't hit lenux-next yet, so here is interdiff
> output)
>
> 	Cyrill
> ---
> diff -u linux-2.6.git/kernel/kcmp.c linux-2.6.git/kernel/kcmp.c
> --- linux-2.6.git/kernel/kcmp.c
> +++ linux-2.6.git/kernel/kcmp.c
> @@ -44,16 +44,25 @@
>  static struct file *
>  get_file_raw_ptr(struct task_struct *task, unsigned int idx)
>  {
> +	struct files_struct *files;
>  	struct fdtable *fdt;
>  	struct file *file;
>
> -	spin_lock(&task->files->file_lock);
> -	fdt = files_fdtable(task->files);
> +	files = get_files_struct(task);
> +	if (!files)
> +		return NULL;
> +
> +	rcu_read_lock();
> +
> +	fdt = files_fdtable(files);
>  	if (idx < fdt->max_fds)
>  		file = fdt->fd[idx];
>  	else
>  		file = NULL;
> -	spin_unlock(&task->files->file_lock);
> +
> +	rcu_read_unlock();
> +
> +	put_files_struct(files);

I think this should work. Or you can simply do

	struct file *file = NULL;

	task_lock(task);
	rcu_read_lock();
	if (task->files)
		file = fcheck_files(task->files, idx);
	rcu_read_unlock();
	task_unlock(task);

Oleg.

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