GitLab

The lockdep warning below detects a possible A->B/B->A locking
dependency of mm->mmap_sem and dcookie_mutex. The order in
sync_buffer() is mm->mmap_sem/dcookie_mutex, while in
sys_lookup_dcookie() it is vice versa.

Fixing it in sys_lookup_dcookie() by unlocking dcookie_mutex before
copy_to_user().

oprofiled/4432 is trying to acquire lock:
 (&mm->mmap_sem){++++++}, at: [<ffffffff810b444b>] might_fault+0x53/0xa3

but task is already holding lock:
 (dcookie_mutex){+.+.+.}, at: [<ffffffff81124d28>] sys_lookup_dcookie+0x45/0x149

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (dcookie_mutex){+.+.+.}:
       [<ffffffff8106557f>] lock_acquire+0xf8/0x11e
       [<ffffffff814634f0>] mutex_lock_nested+0x63/0x309
       [<ffffffff81124e5c>] get_dcookie+0x30/0x144
       [<ffffffffa0000fba>] sync_buffer+0x196/0x3ec [oprofile]
       [<ffffffffa0001226>] task_exit_notify+0x16/0x1a [oprofile]
       ...