linux-kernel - [patch] mm: fix locking, inotify_read's ev_mutex vs do_page_fault's mmap

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Date:	Sun, 14 Sep 2008 10:06:31 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: [patch] mm: fix locking, inotify_read's ev_mutex vs
	do_page_fault's mmap_sem...


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> > [ 6460.634452] 
> > [ 6460.634465] =======================================================
> > [ 6460.634494] [ INFO: possible circular locking dependency detected ]
> > [ 6460.634517] 2.6.27-rc6-tip-00290-g8e229c3-dirty #1
> > [ 6460.634535] -------------------------------------------------------
> > [ 6460.634555] gdm-simple-gree/4778 is trying to acquire lock:
> > [ 6460.634574]  (&mm->mmap_sem){----}, at: [<c018fe33>] might_fault+0x36/0x73
> > [ 6460.634639] 
> > [ 6460.634645] but task is already holding lock:
> > [ 6460.634662]  (&dev->ev_mutex){--..}, at: [<c01c7a76>] inotify_read+0xd8/0x16e
> > [ 6460.634715] 
> > [ 6460.634721] which lock already depends on the new lock.
> 
> Yes, there's a thread in my intray called "inotify_read's ev_mutex vs 
> do_page_fault's mmap_sem...".  It's a bit flakey-looking, but there's 
> a patch in there.

ah, thx. I picked up the patch into tip/out-of-tree. (see below for a 
tided up changelog) Please queue it up as v2.6.27 material. (i'll report 
it if anything breaks due to the patch)

	Ingo

--------------->
>From 1eb0a42e4eb3283521ee1de99adbf567874b622f Mon Sep 17 00:00:00 2001
From: Nick Piggin <nickpiggin@...oo.com.au>
Date: Thu, 11 Sep 2008 06:12:51 +1000
Subject: [PATCH] mm: fix locking, inotify_read's ev_mutex vs do_page_fault's mmap_sem...

Daniel J Blueman wrote:
> I observed this locking violation [1] while gnome-panel was loading;
> this was previously reported at
> http://uwsg.iu.edu/hypermail/linux/kernel/0806.3/2881.html .

Fix inotify lock order reversal with mmap_sem due to holding locks over
copy_to_user.

Reported-by: "Daniel J Blueman" <daniel.blueman@...il.com>
Signed-off-by: Nick Piggin <npiggin@...e.de>
Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 fs/inotify_user.c            |   27 ++++++++++++++++++++-------
 include/asm-x86/uaccess_64.h |    1 +
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/fs/inotify_user.c b/fs/inotify_user.c
index 6024942..d85c7d9 100644
--- a/fs/inotify_user.c
+++ b/fs/inotify_user.c
@@ -323,7 +323,7 @@ out:
 }
 
 /*
- * remove_kevent - cleans up and ultimately frees the given kevent
+ * remove_kevent - cleans up the given kevent
  *
  * Caller must hold dev->ev_mutex.
  */
@@ -334,7 +334,13 @@ static void remove_kevent(struct inotify_device *dev,
 
 	dev->event_count--;
 	dev->queue_size -= sizeof(struct inotify_event) + kevent->event.len;
+}
 
+/*
+ * free_kevent - frees the given kevent.
+ */
+static void free_kevent(struct inotify_kernel_event *kevent)
+{
 	kfree(kevent->name);
 	kmem_cache_free(event_cachep, kevent);
 }
@@ -350,6 +356,7 @@ static void inotify_dev_event_dequeue(struct inotify_device *dev)
 		struct inotify_kernel_event *kevent;
 		kevent = inotify_dev_get_event(dev);
 		remove_kevent(dev, kevent);
+		free_kevent(kevent);
 	}
 }
 
@@ -433,17 +440,15 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 	dev = file->private_data;
 
 	while (1) {
-		int events;
 
 		prepare_to_wait(&dev->wq, &wait, TASK_INTERRUPTIBLE);
 
 		mutex_lock(&dev->ev_mutex);
-		events = !list_empty(&dev->events);
-		mutex_unlock(&dev->ev_mutex);
-		if (events) {
+		if (!list_empty(&dev->events)) {
 			ret = 0;
 			break;
 		}
+		mutex_unlock(&dev->ev_mutex);
 
 		if (file->f_flags & O_NONBLOCK) {
 			ret = -EAGAIN;
@@ -462,7 +467,6 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 	if (ret)
 		return ret;
 
-	mutex_lock(&dev->ev_mutex);
 	while (1) {
 		struct inotify_kernel_event *kevent;
 
@@ -481,6 +485,13 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 			}
 			break;
 		}
+		remove_kevent(dev, kevent);
+
+		/*
+		 * Must perform the copy_to_user outside the mutex in order
+		 * to avoid a lock order reversal with mmap_sem.
+		 */
+		mutex_unlock(&dev->ev_mutex);
 
 		if (copy_to_user(buf, &kevent->event, event_size)) {
 			ret = -EFAULT;
@@ -498,7 +509,9 @@ static ssize_t inotify_read(struct file *file, char __user *buf,
 			count -= kevent->event.len;
 		}
 
-		remove_kevent(dev, kevent);
+		free_kevent(kevent);
+
+		mutex_lock(&dev->ev_mutex);
 	}
 	mutex_unlock(&dev->ev_mutex);
 
diff --git a/include/asm-x86/uaccess_64.h b/include/asm-x86/uaccess_64.h
index 515d4dc..45806d6 100644
--- a/include/asm-x86/uaccess_64.h
+++ b/include/asm-x86/uaccess_64.h
@@ -7,6 +7,7 @@
 #include <linux/compiler.h>
 #include <linux/errno.h>
 #include <linux/prefetch.h>
+#include <linux/lockdep.h>
 #include <asm/page.h>
 
 /*
--
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