Re: bdi: lockdep warning in bdi_queue_work

[Jan Kara <jack@suse.cz>]

  Hello,

On Fri 04-04-14 18:06:37, Sasha Levin wrote:
> While fuzzing with trinity inside a KVM tools guest running the latest -next
> kernel I've stumbled on the following:
> 
> [  323.192041] INFO: trying to register non-static key.
> [  323.193083] the code is fine but needs lockdep annotation.
> [  323.193949] turning off the locking correctness validator.
> [  323.194687] CPU: 15 PID: 21793 Comm: trinity-c94 Not tainted 3.14.0-next-20140403-sasha-00019-g7474aa9-dirty #376
> [  323.196300]  0000000000000000 ffff8804d9067cf8 ffffffff954bfb2f 0000000000000000
> [  323.197522]  ffffffff99378b10 ffff8804d9067df8 ffffffff921c3912 ffff88082bddaeb0
> [  323.198879]  ffff880800000000 ffff880400000001 ffffffff00000000 ffff8804d9067d48
> [  323.200063] Call Trace:
> [  323.200487] dump_stack (lib/dump_stack.c:52)
> [  323.200581] __lock_acquire (kernel/locking/lockdep.c:743 kernel/locking/lockdep.c:3078)
> [  323.200581] ? __slab_alloc (mm/slub.c:2385 (discriminator 2))
> [  323.200581] ? __lock_acquire (kernel/locking/lockdep.c:3189)
> [  323.200581] ? kvm_clock_read (arch/x86/include/asm/preempt.h:90 arch/x86/kernel/kvmclock.c:86)
> [  323.200581] lock_acquire (arch/x86/include/asm/current.h:14 kernel/locking/lockdep.c:3602)
> [  323.200581] ? bdi_queue_work (arch/x86/include/asm/bitops.h:313 fs/fs-writeback.c:108)
> [  323.200581] _raw_spin_lock_bh (include/linux/spinlock_api_smp.h:136 kernel/locking/spinlock.c:175)
> [  323.200581] ? bdi_queue_work (arch/x86/include/asm/bitops.h:313 fs/fs-writeback.c:108)
> [  323.200581] bdi_queue_work (arch/x86/include/asm/bitops.h:313 fs/fs-writeback.c:108)
> [  323.200581] __bdi_start_writeback (fs/fs-writeback.c:141)
> [  323.200581] wakeup_flusher_threads (fs/fs-writeback.c:1077)
> [  323.200581] ? wakeup_flusher_threads (include/linux/rcupdate.h:800 fs/fs-writeback.c:1076)
> [  323.200581] ? syscall_trace_enter (include/linux/context_tracking.h:27 arch/x86/kernel/ptrace.c:1461)
> [  323.200581] sys_sync (fs/sync.c:107)
> [  323.200581] tracesys (arch/x86/kernel/entry_64.S:749)
  Thanks for report. This is really strange. The complaint is apparently
about bdi->wb_lock. But that is properly initialized with spin_lock_init()
when bdi is created so I don't see how we could see a non-static key there.
Can you reproduce this? Can you tell what the non-static key was?

I presume something bad could happen if someone was freeing the bdi while
we are looking at it. And given bdi should be RCU freed, that could happen
if someone forgot to free the bdi structure using RCU. So to identify that
better, can you dump 'bdi->name' for the bdi which triggers this?

							Thanks
								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR