Messages in this thread Patch in this message | | | Subject | [patch] inotify: fix deadlock found by lockdep | From | Arjan van de Ven <> | Date | Mon, 24 Jul 2006 19:21:10 +0200 |
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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;
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