[patch] inotify: fix deadlock found by lockdep

Arjan van de Ven
2006-07-24T17:22:15Z

Subject: [patch] inotify: fix deadlock found by lockdep
From: Arjan van de Ven <arjan@linux.intel.com>

This is a real deadlock, a nice complex one:
(warning: long explanation follows so that Andrew can have a complete
patch description)

it's an ABCDA deadlock:

A iprune_mutex
B inode->inotify_mutex
C ih->mutex
D dev->ev_mutex

The AB relationship comes straight from invalidate_inodes()

int invalidate_inodes(struct super_block * sb)
{
int busy;
LIST_HEAD(throw_away);

mutex_lock(&iprune_mutex);
spin_lock(&inode_lock);
inotify_unmount_inodes(&sb->s_inodes);

where inotify_umount_inodes() takes the
mutex_lock(&inode->inotify_mutex);

The BC relationship comes directly from inotify_find_update_watch():
s32 inotify_find_update_watch(struct inotify_handle *ih, struct inode *inode,
u32 mask)
{
...
mutex_lock(&inode->inotify_mutex);
mutex_lock(&ih->mutex);

The CD relationship comes from inotify_rm_wd:
inotify_rm_wd does
mutex_lock(&inode->inotify_mutex);
mutex_lock(&ih->mutex)
and then calls inotify_remove_watch_locked() which calls
notify_dev_queue_event() which does
mutex_lock(&dev->ev_mutex);

(this strictly is a BCD relationship)

The DA relationship comes from the most interesting part:

[<ffffffff8022d9f2>] shrink_icache_memory+0x42/0x270
[<ffffffff80240dc4>] shrink_slab+0x11d/0x1c9
[<ffffffff802b5104>] try_to_free_pages+0x187/0x244
[<ffffffff8020efed>] __alloc_pages+0x1cd/0x2e0
[<ffffffff8025e1f8>] cache_alloc_refill+0x3f8/0x821
[<ffffffff8020a5e5>] kmem_cache_alloc+0x85/0xcb
[<ffffffff802db027>] kernel_event+0x2e/0x122
[<ffffffff8021d61c>] inotify_dev_queue_event+0xcc/0x140

inotify_dev_queue_event schedules a kernel_event which does a
kmem_cache_alloc( , GFP_KERNEL) which may try to shrink slabs, including
the inode cache .. which then takes iprune_mutex.

And voila, there is an AB, a BC, a CD relationship (even a direct BCD),
and also now a DA relationship -> a circular type AB-BA deadlock but
involving 4 locks.

The solution is simple: kernel_event() is NOT allowed to use GFP_KERNEL,
but must use GFP_NOFS to not cause recursion into the VFS.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>

Index: linux-2.6.18-rc1/fs/inotify_user.c
===================================================================
--- linux-2.6.18-rc1.orig/fs/inotify_user.c
+++ linux-2.6.18-rc1/fs/inotify_user.c
@@ -187,7 +187,7 @@ static struct inotify_kernel_event * ker
{
struct inotify_kernel_event *kevent;

- kevent = kmem_cache_alloc(event_cachep, GFP_KERNEL);
+ kevent = kmem_cache_alloc(event_cachep, GFP_NOFS);
if (unlikely(!kevent))
return NULL;