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

From: Filipe Manana
Date: Wed Nov 04 2020 - 14:54:51 EST

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In reply to: Filipe Manana: "Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")"
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On 04/11/20 09:49, Filipe Manana wrote:
>
>
> On 04/11/20 03:44, Boqun Feng wrote:
>> On Wed, Nov 04, 2020 at 10:22:36AM +0800, Boqun Feng wrote:
>>> On Tue, Nov 03, 2020 at 07:44:29PM +0000, Filipe Manana wrote:
>>>>
>>>>
>>>> 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. Although I think it still means that we have a lock issue when
>>> running xfstests (because we don't know why debug_locks gets cleared),
>>
>> I might find a place where we could turn lockdep off silently:
>>
>> in print_circular_bug(), we turn off lockdep via
>> debug_locks_off_graph_unlock(), and then we try to save the trace for
>> lockdep splat, however, if we use up the stack_trace buffer (i.e.
>> nr_stack_trace_entries), save_trace() will return NULL and we return
>> silently.
>>
>> Filipe, in order to check whethter that happens, could you share me your
>> /proc/lockdep_stats after the full set of xfstests is finished?
>
> Here it is:
>
> $ cat /proc/lockdep_stats
> lock-classes: 1831 [max: 8192]
> direct dependencies: 17774 [max: 32768]
> indirect dependencies: 75662
> all direct dependencies: 325284
> dependency chains: 34223 [max: 65536]
> dependency chain hlocks used: 158129 [max: 327680]
> dependency chain hlocks lost: 0
> in-hardirq chains: 57
> in-softirq chains: 658
> in-process chains: 33508
> stack-trace entries: 160748 [max: 524288]
> number of stack traces: 9237
> number of stack hash chains: 7076
> combined max dependencies: 1280780998
> hardirq-safe locks: 43
> hardirq-unsafe locks: 1337
> softirq-safe locks: 179
> softirq-unsafe locks: 1236
> irq-safe locks: 187
> irq-unsafe locks: 1337
> hardirq-read-safe locks: 2
> hardirq-read-unsafe locks: 209
> softirq-read-safe locks: 9
> softirq-read-unsafe locks: 204
> irq-read-safe locks: 9
> irq-read-unsafe locks: 209
> uncategorized locks: 274
> unused locks: 0
> max locking depth: 15
> max bfs queue depth: 337
> debug_locks: 0
>
> zapped classes: 2278
> zapped lock chains: 17915
> large chain blocks: 1
>
> (That's the result after running all fstests with the previous one line
> patch you sent.)
>
> My kernel .config: https://pastebin.com/4xEMvLJ9
>
> I'll try the debugging patch and let you know the results. It will take
> some 4 hours or so to get back with the result.

Ok, so I ran 5.10-rc2 plus your two patches (the fix and the debug one):

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index b71ad8d9f1c9..b31d4ad482c7 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -539,8 +539,10 @@ static struct lock_trace *save_trace(void)
LOCK_TRACE_SIZE_IN_LONGS;

if (max_entries <= 0) {
- if (!debug_locks_off_graph_unlock())
+ if (!debug_locks_off_graph_unlock()) {
+ WARN_ON_ONCE(1);
return NULL;
+ }

print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
dump_stack();
@@ -5465,7 +5467,7 @@ 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);

With 3 runs of all fstests, the WARN_ON_ONCE(1) wasn't triggered.
Unexpected, right?

Should I try something else?

Thanks.

>
> Thanks!
>
>>
>> Alternatively, it's also helpful if you can try the following debug
>> diff, with teh full set of xfstests:
>>
>> Thanks! Just trying to understand the real problem.
>>
>> Regards,
>> Boqun
>>
>> -------------->8
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index b71ad8d9f1c9..9ae3e089e5c0 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -539,8 +539,10 @@ static struct lock_trace *save_trace(void)
>> LOCK_TRACE_SIZE_IN_LONGS;
>>
>> if (max_entries <= 0) {
>> - if (!debug_locks_off_graph_unlock())
>> + if (!debug_locks_off_graph_unlock()) {
>> + WARN_ON_ONCE(1);
>> return NULL;
>> + }
>>
>> print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
>> dump_stack();
>>
>>> I guess I will have to reproduce this myself for further analysis, could
>>> you share you .config?
>>>
>>> Anyway, I think this fix still makes a bit sense, I will send a proper
>>> patch so that the problem won't block fs folks.
>>>
>>> Regards,
>>> Boqun
>>>
>>>> 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.
>>>>>
>>

Next message: Nick Desaulniers: "Re: [PATCH] MAINTAINERS: mark LLVM as maintained rather than supported"
Previous message: Ahmed S. Darwish: "Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork"
In reply to: Filipe Manana: "Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")"
Next in thread: Boqun Feng: "Re: possible lockdep regression introduced by 4d004099a668 ("lockdep: Fix lockdep recursion")"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]