Re: [PATCH] debugobjects: Move printk out of db lock critical sections

[Waiman Long <longman@redhat.com>]

On 12/12/2018 11:35 PM, Dmitry Safonov wrote:
> Hi Waiman,
>
> On 12/12/18 10:28 PM, Waiman Long wrote:
>> The db->lock is a raw spinlock and so the lock hold time is supposed
>> to be short. This will not be the case when printk() is being involved
>> in some of the critical sections. In order to avoid the long hold time,
>> in case some messages need to be printed, the debug_object_is_on_stack()
>> and debug_print_object() calls are now moved out of those critical
>> sections.
>>
>> Holding the db->lock while calling printk() may lead to deadlock if
>> printk() somehow requires the allocation/freeing of debug object that
>> happens to be in the same hash bucket or a circular lock dependency
>> warning from lockdep as reported in https://lkml.org/lkml/2018/12/11/143.
>>
>> [   87.209665] WARNING: possible circular locking dependency detected
>> [   87.210547] 4.20.0-rc4-00057-gc96cf92 #1 Tainted: G        W
>> [   87.211449] ------------------------------------------------------
>> [   87.212405] getty/519 is trying to acquire lock:
>> [   87.213074] (____ptrval____) (&obj_hash[i].lock){-.-.}, at: debug_check_no_obj_freed+0xb4/0x302
>> [   87.214343]
>> [   87.214343] but task is already holding lock:
>> [   87.215174] (____ptrval____) (&port_lock_key){-.-.}, at: uart_shutdown+0x3a3/0x4e2
>> [   87.216260]
>> [   87.216260] which lock already depends on the new lock.
>>
>> This patch was also found to be able to fix a boot hanging problem
>> when the initramfs image was switched on after a debugobjects splat
>> from the EFI code.
>>
>> Signed-off-by: Waiman Long <longman@redhat.com>
>> ---
> I've tried to review it and found minor issues like missed
> debug_object_is_on_stack() for initializing already active object.
>
> But than I come to opinion that it's just generally unsafe:
> debug_obj life-time is protected by bucket's spin_lock.
> Check the conditions when free_object() is being called.

The bucket lock is for protecting the insertion and deletion of
debug_obj to/from the bucket list as well as searching within the bucket
list. It has nothing to do with the life time of the debug_obj itself.

Cheers,
Longman