Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

[yangerkun <yangerkun@huawei.com>]

On 2020/3/17 9:41, yangerkun wrote:
> 
> 
> On 2020/3/17 1:26, Linus Torvalds wrote:
>> On Mon, Mar 16, 2020 at 4:07 AM Jeff Layton <jlayton@kernel.org> wrote:
>>>
>>>
>>> +       /*
>>> +        * If fl_blocker is NULL, it won't be set again as this 
>>> thread "owns"
>>> +        * the lock and is the only one that might try to claim the 
>>> lock.
>>> +        * Because fl_blocker is explicitly set last during a delete, 
>>> it's
>>> +        * safe to locklessly test to see if it's NULL. If it is, 
>>> then we know
>>> +        * that no new locks can be inserted into its 
>>> fl_blocked_requests list,
>>> +        * and we can therefore avoid doing anything further as long 
>>> as that
>>> +        * list is empty.
>>> +        */
>>> +       if (!smp_load_acquire(&waiter->fl_blocker) &&
>>> +           list_empty(&waiter->fl_blocked_requests))
>>> +               return status;
>>
>> Ack. This looks sane to me now.
>>
>> yangerkun - how did you find the original problem?\
> 
> While try to fix CVE-2019-19769, add some log in __locks_wake_up_blocks 
> help me to rebuild the problem soon. This help me to discern the problem 
> soon.
> 
>>
>> Would you mind using whatever stress test that caused commit
>> 6d390e4b5d48 ("locks: fix a potential use-after-free problem when
>> wakeup a waiter") with this patch? And if you did it analytically,
>> you're a champ and should look at this patch too!
> 
> I will try to understand this patch, and if it's looks good to me, will 
> do the performance test!

This patch looks good to me, with this patch, the bug '6d390e4b5d48 
("locks: fix a potential use-after-free problem when wakeup a waiter")' 
describes won't happen again. Actually, I find that syzkaller has report 
this bug before[1], and the log of it can help us to reproduce it with 
some latency in __locks_wake_up_blocks!

Also, some ltp testcases describes in [2] pass too with the patch!

For performance test, I have try to understand will-it-scale/lkp, but it 
seem a little complex to me, and may need some more time. So, Rong Chen, 
can you help to do this? Or the results may come a little later...

Thanks,
----
[1] https://syzkaller.appspot.com/bug?extid=922689db06e57b69c240
[2] https://lkml.org/lkml/2020/3/11/578