[PATCH] make lockdep happy with r/o bind mounts

From: Dave Hansen
Date: Mon Sep 24 2007 - 13:47:21 EST

With the r/o bind mount patches, we can have as many
spinlocks nested as there are CPUs on the system.
Lockdep freaks out after 8.

So, create a new lockdep class of locks for the
mnt_writer spinlocks, and initialize each of the
cpu locks to be in a different class.

It should shut up warnings like this, while still
allowing some of the lockdep goodness to remain:

[ INFO: possible recursive locking detected ]
2.6.23-rc6 #6
swapper/1 is trying to acquire lock:
(&writer->lock){--..}, at: [<c016c094>] lock_and_coalesce_cpu_mnt_writer_counts+0x29/0x5c

but task is already holding lock:
(&writer->lock){--..}, at: [<c016c094>] lock_and_coalesce_cpu_mnt_writer_counts+0x29/0x5c

other info that might help us debug this:
1 lock held by swapper/1:
#0: (&writer->lock){--..}, at: [<c016c094>] lock_and_coalesce_cpu_mnt_writer_counts+0x29/0x5c

stack backtrace:
[<c01034ab>] show_trace_log_lvl+0x19/0x2e
[<c01034d2>] show_trace+0x12/0x14
[<c01035be>] dump_stack+0x14/0x16
[<c0130ab2>] print_deadlock_bug+0xb7/0xc1
[<c0130afc>] check_deadlock+0x40/0x54
[<c013100a>] validate_chain+0x26e/0x319
[<c0132cca>] __lock_acquire+0x6c4/0x731
[<c01332da>] lock_acquire+0x7a/0x94
[<c031bb01>] _spin_lock+0x38/0x62
[<c016c094>] lock_and_coalesce_cpu_mnt_writer_counts+0x29/0x5c
[<c016c5f8>] mntput_no_expire+0x38/0xad
[<c015fbed>] path_release_on_umount+0x15/0x18
[<c016cc76>] sys_umount+0x67/0x6f
[<c010058a>] name_to_dev_t+0x22a/0x24c
[<c0427207>] prepare_namespace+0x89/0x187
[<c0426cdd>] kernel_init+0x9a/0xa3
[<c010336f>] kernel_thread_helper+0x7/0x10

Signed-off-by: Dave Hansen <haveblue@xxxxxxxxxx>

lxc-dave/fs/namespace.c | 2 ++
1 file changed, 2 insertions(+)

diff -puN fs/namespace.c~make-lockdep-happy-with-ro-bind-mounts fs/namespace.c
--- lxc/fs/namespace.c~make-lockdep-happy-with-ro-bind-mounts 2007-09-24 10:32:23.000000000 -0700
+++ lxc-dave/fs/namespace.c 2007-09-24 10:41:37.000000000 -0700
@@ -116,6 +116,7 @@ struct mnt_writer {
* must be ordered by cpu number.
spinlock_t lock;
+ struct lock_class_key lock_class; /* compiles out with !lockdep */
unsigned long count;
struct vfsmount *mnt;
} ____cacheline_aligned_in_smp;
@@ -127,6 +128,7 @@ static int __init init_mnt_writers(void)
for_each_possible_cpu(cpu) {
struct mnt_writer *writer = &per_cpu(mnt_writers, cpu);
+ lockdep_set_class(&writer->lock, &writer->lock_class);
writer->count = 0;
return 0;
diff -puN include/linux/lockdep.h~make-lockdep-happy-with-ro-bind-mounts include/linux/lockdep.h
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