Re: [PATCH] mm: Fix possible deadlock in console_trylock_spinning

[Waiman Long <llong@redhat.com>]

On 8/1/25 6:33 PM, Andrew Morton wrote:
> On Wed, 30 Jul 2025 17:49:14 +0800 Gu Bowen <gubowen5@huawei.com> wrote:
>
>> kmemleak_scan_thread() invokes scan_block() which may invoke a nomal
>> printk() to print warning message. This can cause a deadlock in the
>> scenario reported below:
>>
>>         CPU0                    CPU1
>>         ----                    ----
>>    lock(kmemleak_lock);
>>                                 lock(&port->lock);
>>                                 lock(kmemleak_lock);
>>    lock(console_owner);
>>
>> To solve this problem, switch to printk_safe mode before printing warning
>> message, this will redirect all printk()-s to a special per-CPU buffer,
>> which will be flushed later from a safe context (irq work), and this
>> deadlock problem can be avoided.
>>
>> Our syztester report the following lockdep error:
>>
>> ...
>>
>> index 4801751cb6b6..d322897a1de1 100644
>> --- a/mm/kmemleak.c
>> +++ b/mm/kmemleak.c
>> @@ -390,9 +390,11 @@ static struct kmemleak_object *lookup_object(unsigned long ptr, int alias)
>>   		else if (object->pointer == ptr || alias)
>>   			return object;
>>   		else {
>> +			__printk_safe_enter();
>>   			kmemleak_warn("Found object by alias at 0x%08lx\n",
>>   				      ptr);
>>   			dump_object_info(object);
>> +			__printk_safe_exit();
>>   			break;
>>   		}
>>   	}
> Thanks.
>
> There have been a few kmemleak locking fixes lately.
>
> I believe this fix is independent from the previous ones:
>
> https://lkml.kernel.org/r/20250731-kmemleak_lock-v1-1-728fd470198f@debian.org
> https://lkml.kernel.org/r/20250728190248.605750-1-longman@redhat.com
>
> But can people please check?

I believe that __printk_safe_enter()/_printk_safe_exit() are for printk 
internal use only. The proper API to use should be 
printk_deferred_enter()/printk_deferred_exit() if we want to deferred 
the printing. Since kmemleak_lock will have been acquired with irq 
disabled, it meets the condition that printk_deferred_*() APIs can be used.

Cheers,
Longman