Re: [PATCH v2] mm/page_isolation: fix a deadlock with printk()

[Christian Borntraeger]

Adding Peter Oberparleiter.
Peter, can you have a look?

On 08.10.19 10:27, Michal Hocko wrote:
> On Tue 08-10-19 09:43:57, Petr Mladek wrote:
>> On Mon 2019-10-07 16:49:37, Michal Hocko wrote:
>>> [Cc s390 maintainers - the lockdep is http://lkml.kernel.org/r/1570228005-24979-1-git-send-email-cai@xxxxxx
>>>  Petr has explained it is a false positive
>>>  http://lkml.kernel.org/r/20191007143002.l37bt2lzqtnqjqxu@xxxxxxxxxxxxxxx]
>>> On Mon 07-10-19 16:30:02, Petr Mladek wrote:
>>> [...]
>>>> I believe that it cannot really happen because:
>>>>
>>>> 	static int __init
>>>> 	sclp_console_init(void)
>>>> 	{
>>>> 	[...]
>>>> 		rc = sclp_rw_init();
>>>> 	[...]
>>>> 		register_console(&sclp_console);
>>>> 		return 0;
>>>> 	}
>>>>
>>>> sclp_rw_init() is called before register_console(). And
>>>> console_unlock() will never call sclp_console_write() before
>>>> the console is registered.
>>>>
>>>> AFAIK, lockdep only compares existing chain of locks. It does
>>>> not know about console registration that would make some
>>>> code paths mutually exclusive.
>>>>
>>>> I believe that it is a false positive. I do not know how to
>>>> avoid this lockdep report. I hope that it will disappear
>>>> by deferring all printk() calls rather soon.
>>>
>>> Thanks a lot for looking into this Petr. I have also checked the code
>>> and I really fail to see why the allocation has to be done under the
>>> lock in the first place. sclp_read_sccb and sclp_init_sccb are global
>>> variables but I strongly suspect that they need a synchronization during
>>> early init, callbacks are registered only later IIUC:
>>
>> Good idea. It would work when the init function is called only once.
>> But see below.
>>
>>> diff --git a/drivers/s390/char/sclp.c b/drivers/s390/char/sclp.c
>>> index d2ab3f07c008..4b1c033e3255 100644
>>> --- a/drivers/s390/char/sclp.c
>>> +++ b/drivers/s390/char/sclp.c
>>> @@ -1169,13 +1169,13 @@ sclp_init(void)
>>>  	unsigned long flags;
>>>  	int rc = 0;
>>>  
>>> +	sclp_read_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
>>> +	sclp_init_sccb = (void *) __get_free_page(GFP_ATOMIC | GFP_DMA);
>>>  	spin_lock_irqsave(&sclp_lock, flags);
>>>  	/* Check for previous or running initialization */
>>>  	if (sclp_init_state != sclp_init_state_uninitialized)
>>>  		goto fail_unlock;
>>
>> It seems that sclp_init() could be called several times in parallel.
>> I see it called from sclp_register() and sclp_initcall().
> 
> Interesting. Something for s390 people to answer I guess.
> Anyway, this should be quite trivial to workaround by a cmpxch or alike.
>