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    SubjectRe: [PATCH] make lockdep happy with r/o bind mounts
    On Mon, 24 Sep 2007 10:46:58 -0700 Dave Hansen <haveblue@us.ibm.com>
    wrote:

    >
    > 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@us.ibm.com>

    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>

    > ---
    >
    > 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);
    > spin_lock_init(&writer->lock);
    > + 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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