Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")

[Filipe Manana]

On 03/11/20 14:08, Boqun Feng wrote:
> Hi Filipe,
> 
> On Mon, Oct 26, 2020 at 11:26:49AM +0000, Filipe Manana wrote:
>> Hello,
>>
>> I've recently started to hit a warning followed by tasks hanging after
>> attempts to freeze a filesystem. A git bisection pointed to the
>> following commit:
>>
>> commit 4d004099a668c41522242aa146a38cc4eb59cb1e
>> Author: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Date:   Fri Oct 2 11:04:21 2020 +0200
>>
>>     lockdep: Fix lockdep recursion
>>
>> This happens very reliably when running all xfstests with lockdep
>> enabled, and the tested filesystem is btrfs (haven't tried other
>> filesystems, but it shouldn't matter). The warning and task hangs always
>> happen at either test generic/068 or test generic/390, and (oddly)
>> always have to run all tests for it to trigger, running those tests
>> individually on an infinite loop doesn't seem to trigger it (at least
>> for a couple hours).
>>
>> The warning triggered is at fs/super.c:__sb_start_write() which always
>> results later in several tasks hanging on a percpu rw_sem:
>>
>> https://pastebin.com/qnLvf94E
>>
> 
> In your dmesg, I see line:
> 
> 	[ 9304.920151] INFO: lockdep is turned off.
> 
> , that means debug_locks is 0, that usually happens when lockdep find a
> problem (i.e. a deadlock) and it turns itself off, because a problem is
> found and it's pointless for lockdep to continue to run.
> 
> And I haven't found a lockdep splat in your dmesg, do you have a full
> dmesg so that I can have a look?
> 
> This may be relevant because in commit 4d004099a66, we have
> 
> 	@@ -5056,13 +5081,13 @@ noinstr int lock_is_held_type(const struct lockdep_map *lock, int read)
> 		unsigned long flags;
> 		int ret = 0;
> 
> 	-       if (unlikely(current->lockdep_recursion))
> 	+       if (unlikely(!lockdep_enabled()))
> 			return 1; /* avoid false negative lockdep_assert_held() */
> 
> before this commit lock_is_held_type() and its friends may return false
> if debug_locks==0, after this commit lock_is_held_type() and its friends
> will always return true if debug_locks == 0. That could cause the
> behavior here.
> 
> In case I'm correct, the following "fix" may be helpful. 
> 
> Regards,
> Boqun
> 
> ----------8
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 3e99dfef8408..c0e27fb949ff 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -5471,7 +5464,7 @@ noinstr int lock_is_held_type(const struct lockdep_map *lock, int read)
>  	unsigned long flags;
>  	int ret = 0;
>  
> -	if (unlikely(!lockdep_enabled()))
> +	if (unlikely(debug_locks && !lockdep_enabled()))
>  		return 1; /* avoid false negative lockdep_assert_held() */
>  
>  	raw_local_irq_save(flags);

Boqun, the patch fixes the problem for me!
You can have Tested-by: Filipe Manana <fdmanana@xxxxxxxx>

Thanks!

> 
> 
> 
>> What happens is percpu_rwsem_is_held() is apparently returning a false
>> positive, so this makes __sb_start_write() do a
>> percpu_down_read_trylock() on a percpu_rw_sem at a higher level, which
>> is expected to always succeed, because if the calling task is holding a
>> freeze percpu_rw_sem at level 1, it's supposed to be able to try_lock
>> the semaphore at level 2, since the freeze semaphores are always
>> acquired by increasing level order.
>>
>> But the try_lock fails, it triggers the warning at __sb_start_write(),
>> then its caller sb_start_pagefault() ignores the return value and
>> callers such as btrfs_page_mkwrite() make the assumption the freeze
>> semaphore was taken, proceed to do their stuff, and later call
>> sb_end_pagefault(), which which will do an up_read() on the percpu_rwsem
>> at level 2 despite not having not been able to down_read() the
>> semaphore. This obviously corrupts the semaphore's read_count state, and
>> later causes any task trying to down_write() it to hang forever.
>>
>> After such a hang I ran a drgn script to confirm it:
>>
>> $ cat dump_freeze_sems.py
>> import sys
>> import drgn
>> from drgn import NULL, Object, cast, container_of, execscript, \
>>     reinterpret, sizeof
>> from drgn.helpers.linux import *
>>
>> mnt_path = b'/home/fdmanana/btrfs-tests/scratch_1'
>>
>> mnt = None
>> for mnt in for_each_mount(prog, dst = mnt_path):
>>     pass
>>
>> if mnt is None:
>>     sys.stderr.write(f'Error: mount point {mnt_path} not found\n')
>>     sys.exit(1)
>>
>> def dump_sem(level_enum):
>>     level = level_enum.value_()
>>     sem = mnt.mnt.mnt_sb.s_writers.rw_sem[level - 1]
>>     print(f'freeze semaphore at level {level}, {str(level_enum)}')
>>     print(f'    block {sem.block.counter.value_()}')
>>     for i in for_each_possible_cpu(prog):
>>         read_count = per_cpu_ptr(sem.read_count, i)
>>         print(f'    read_count at cpu {i} = {read_count}')
>>     print()
>>
>> # dump semaphore read counts for all freeze levels (fs.h)
>> dump_sem(prog['SB_FREEZE_WRITE'])
>> dump_sem(prog['SB_FREEZE_PAGEFAULT'])
>> dump_sem(prog['SB_FREEZE_FS'])
>>
>>
>> $ drgn dump_freeze_sems.py
>> freeze semaphore at level 1, (enum <anonymous>)SB_FREEZE_WRITE
>>     block 1
>>     read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c74 = 3
>>     read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c74 = 4294967293
>>     read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c74 = 3
>>     read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c74 = 4294967293
>>
>> freeze semaphore at level 2, (enum <anonymous>)SB_FREEZE_PAGEFAULT
>>     block 1
>>     read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c78 = 0
>>     read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c78 = 4294967295
>>     read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c78 = 0
>>     read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c78 = 0
>>
>> freeze semaphore at level 3, (enum <anonymous>)SB_FREEZE_FS
>>     block 0
>>     read_count at cpu 0 = *(unsigned int *)0xffffc2ec3ee00c7c = 0
>>     read_count at cpu 1 = *(unsigned int *)0xffffc2ec3f200c7c = 0
>>     read_count at cpu 2 = *(unsigned int *)0xffffc2ec3f600c7c = 0
>>     read_count at cpu 3 = *(unsigned int *)0xffffc2ec3fa00c7c = 0
>>
>> At levels 1 and 3, read_count sums to 0, so it's fine, but at level 2 it
>> sums to -1. The system remains like that for hours at least, with no
>> progress at all.
>>
>> Is there a known regression with that lockdep commit?
>> Anything I can do to help debug it in case it's not obvious?
>>
>> Thanks.
>