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

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Date:	Wed, 15 Feb 2012 22:32:03 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
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 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);
 
 	return file;
 }
--
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