On 09/11/20 08:44, Boqun Feng wrote:
> Hi Filipe,
>
> On Thu, Nov 05, 2020 at 09:10:12AM +0800, Boqun Feng wrote:
>> On Wed, Nov 04, 2020 at 07:54:40PM +0000, Filipe Manana wrote:
>> [...]
>>>
>>> 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?
>>>
>>
>> Kinda, that means we still don't know why lockdep was turned off.
>>
>>> Should I try something else?
>>>
>>
>> Thanks for trying this. Let me set up the reproducer on my side, and see
>> if I could get more information.
>>
>
> I could hit this with btrfs/187, and when we hit it, lockdep will report
> the deadlock and turn off, and I think this is the root cause for your
> hitting the original problem, I will add some analysis after the lockdep
> splat.
>
> [12295.973309] ============================================
> [12295.974770] WARNING: possible recursive locking detected
> [12295.974770] 5.10.0-rc2-btrfs-next-71 #20 Not tainted
> [12295.974770] --------------------------------------------
> [12295.974770] zsh/701247 is trying to acquire lock:
> [12295.974770] ffff92cef43480b8 (&eb->lock){++++}-{2:2}, at:
> btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770]
> but task is already holding lock:
> [12295.974770] ffff92cef434a038 (&eb->lock){++++}-{2:2}, at:
> btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770]
> other info that might help us debug this:
> [12295.974770] Possible unsafe locking scenario:
>
> [12295.974770] CPU0
> [12295.974770] ----
> [12295.974770] lock(&eb->lock);
> [12295.974770] lock(&eb->lock);
> [12295.974770]
> *** DEADLOCK ***
>
> [12295.974770] May be due to missing lock nesting notation
>
> [12295.974770] 2 locks held by zsh/701247:
> [12295.974770] #0: ffff92cef3d315b0 (&sig->cred_guard_mutex){+.+.}-{3:3},
> at: bprm_execve+0xaa/0x920
> [12295.974770] #1: ffff92cef434a038 (&eb->lock){++++}-{2:2}, at:
> btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770]
> stack backtrace:
> [12295.974770] CPU: 6 PID: 701247 Comm: zsh Not tainted
> 5.10.0-rc2-btrfs-next-71 #20
> [12295.974770] Hardware name: Microsoft Corporation Virtual Machine/Virtual
> Machine, BIOS Hyper-V UEFI Release v4.0 12/17/2019
> [12295.974770] Call Trace:
> [12295.974770] dump_stack+0x8b/0xb0
> [12295.974770] __lock_acquire.cold+0x175/0x2e9
> [12295.974770] lock_acquire+0x15b/0x490
> [12295.974770] ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770] ? read_block_for_search+0xf4/0x350 [btrfs]
> [12295.974770] _raw_read_lock+0x40/0xa0
> [12295.974770] ? btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770] btrfs_tree_read_lock_atomic+0x34/0x140 [btrfs]
> [12295.974770] btrfs_search_slot+0x6ac/0xca0 [btrfs]
> [12295.974770] btrfs_lookup_xattr+0x7d/0xd0 [btrfs]
> [12295.974770] btrfs_getxattr+0x67/0x130 [btrfs]
> [12295.974770] __vfs_getxattr+0x53/0x70
> [12295.974770] get_vfs_caps_from_disk+0x68/0x1a0
> [12295.974770] ? sched_clock_cpu+0x114/0x180
> [12295.974770] cap_bprm_creds_from_file+0x181/0x6c0
> [12295.974770] security_bprm_creds_from_file+0x2a/0x40
> [12295.974770] begin_new_exec+0xf4/0xc40
> [12295.974770] ? load_elf_phdrs+0x6b/0xb0
> [12295.974770] load_elf_binary+0x66b/0x1620
> [12295.974770] ? read_hv_sched_clock_tsc+0x5/0x20
> [12295.974770] ? sched_clock+0x5/0x10
> [12295.974770] ? sched_clock_local+0x12/0x80
> [12295.974770] ? sched_clock_cpu+0x114/0x180
> [12295.974770] bprm_execve+0x3ce/0x920
> [12295.974770] do_execveat_common+0x1b0/0x1f0
> [12295.974770] __x64_sys_execve+0x39/0x50
> [12295.974770] do_syscall_64+0x33/0x80
> [12295.974770] entry_SYSCALL_64_after_hwframe+0x44/0xa9
> [12295.974770] RIP: 0033:0x7f6aaefc13cb
> [12295.974770] Code: 48 3d 00 f0 ff ff 76 e7 f7 d8 64 41 89 00 eb df 0f 1f 80
> 00 00 00 00 f7 d8 64 41 89 00 eb dc f3 0f 1e fa b8 3b 00 00 00 0f 05 <48> 3d
> 01 f0 ff ff 73 01 c3 48 8b 0d 75 4a 0f 00 f7 d8 64 89 01 48
> [12295.974770] RSP: 002b:00007ffd33b54d58 EFLAGS: 00000207 ORIG_RAX:
> 000000000000003b
> [12295.974770] RAX: ffffffffffffffda RBX: 00007f6aaf28bc88 RCX:
> 00007f6aaefc13cb
> [12295.974770] RDX: 00007ffd33b5fd98 RSI: 00007f6aaf28bc88 RDI:
> 00007ffd33b55280
> [12295.974770] RBP: 00007ffd33b54d80 R08: 00007ffd33b54ce0 R09:
> 0000000000000001
> [12295.974770] R10: 0000000000000008 R11: 0000000000000207 R12:
> 00007ffd33b55280
>
> The deadlock senario reported by this splat is as follow:
>
> CPU 0 CPU 2
> ===== =====
> btrfs_search_slot():
> btrfs_tree_read_lock_atomic():
> read_lock(&eb->lock); <a random writer>
> write_lock(&eb->lock);
> // waiting for the lock
> ...
> btrfs_tree_read_lock_atomic():
> read_lock(&eb->lock); // blocked because the fairness.
>
> In short, you can not use nested read_lock() on the same lock. However,
> I'm not sure whether we have the real problem here, because I don't
> think btrfs_search_slot() can pick the same extent buffer twice in
> btrfs_search_slot(). Also copy the author of the code for more
> information.
Ah yes. We have several lockdep splats for which the fixes are not in
5.10-rcs yet. Some may be already in the integration branch [1] and
others not yet in any tree, but can be found in the btrfs mailing list.
For that particular one, it's fixed by the following patch:
https://lore.kernel.org/linux-btrfs/1cee2922a32c305056a9559ccf7aede49777beae.1604591048.git.jo...@toxicpanda.com/
(It belongs to a series)
You'll likely see more different lockdep splats, this is due to a
transition from custom btree locks to rw semaphores that is in progress,
and some preparatory work for that is already in 5.10-rcs.
Btw, I could hit the fs freeze deadlock even when lockdep didn't produce
any previous splat.
Jan was hitting it too with ext4, and Darrick with xfs - though I can't
tell if they hit any lockdep splat before hitting the freeze deadlock.
Thanks for reporting it.
[1] https://github.com/kdave/btrfs-devel/commits/misc-next
>
> Regards,
> Boqun
>
>> Regards,
>> Boqun
>>
>>> Thanks.
>>>
>>>
>
