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5179241521init_mount_tree() mounts the mutable rootfs on top of nullfs via
LOCK_MOUNT_EXACT(). That declares a pinned mountpoint with a cleanup
attribute in the scope of the whole function so the nullfs root inode
lock and namespace_sem are only dropped when init_mount_tree() returns.
This became a problem when the private nullfs instance for kthreads was
added. kern_mount() allocates a new superblock and alloc_super() takes
the new s_umount with SINGLE_DEPTH_NESTING and then shrinker_mutex via
shrinker_alloc(). Doing that with namespace_sem held teaches lockdep the
dependency
namespace_sem -> s_umount/1 -> shrinker_mutex
With CONFIG_SHRINKER_DEBUG shrinker_debugfs_rename() takes the debugfs
directory inode lock under shrinker_mutex every time a block device is
mounted and lock_mount_exact() takes namespace_sem under the inode lock
of the mountpoint for every mount. So mounting anything on debugfs,
e.g. the tracefs automount on /sys/kernel/debug/tracing, closes the
cycle:
WARNING: possible circular locking dependency detected
7.3.0-rc3+ #17 Not tainted
------------------------------------------------------
rasdaemon/4449 is trying to acquire lock:
(namespace_sem){++++}-{4:4}, at: lock_mount_exact+0x4c/0x308
but task is already holding lock:
(&sb->s_type->i_mutex_key#17){++++}-{4:4}, at: lock_mount_exact+0x3c/0x308
which lock already depends on the new lock.
...
Chain exists of:
namespace_sem --> shrinker_mutex --> &sb->s_type->i_mutex_key#17
This can't actually deadlock. init_mount_tree() runs single-threaded
during early boot before any other task exists and nothing allocates a
superblock under namespace_sem after that. But lockdep can't know that
and disables itself for the rest of the boot.
Move mounting the rootfs on top of nullfs into a helper so the locks
are dropped when it returns.
Fixes:
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