Re: possible deadlock in sk_clone_lock

[Mike Kravetz]

Cc: Michal

On 2/26/21 2:44 PM, Shakeel Butt wrote:
> On Fri, Feb 26, 2021 at 2:09 PM syzbot
> <syzbot+506c8a2a115201881d45@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
<snip>
>> other info that might help us debug this:
>>
>>  Possible interrupt unsafe locking scenario:
>>
>>        CPU0                    CPU1
>>        ----                    ----
>>   lock(hugetlb_lock);
>>                                local_irq_disable();
>>                                lock(slock-AF_INET);
>>                                lock(hugetlb_lock);
>>   <Interrupt>
>>     lock(slock-AF_INET);
>>
>>  *** DEADLOCK ***
> 
> This has been reproduced on 4.19 stable kernel as well [1] and there
> is a reproducer as well.
> 
> It seems like sendmsg(MSG_ZEROCOPY) from a buffer backed by hugetlb. I
> wonder if we just need to make hugetlb_lock softirq-safe.
> 
> [1] https://syzkaller.appspot.com/bug?extid=6383ce4b0b8ec575ad93

Thanks Shakeel,

Commit c77c0a8ac4c5 ("mm/hugetlb: defer freeing of huge pages if in non-task
context") attempted to address this issue.  It uses a work queue to
acquire hugetlb_lock if the caller is !in_task().

In another recent thread, there was the suggestion to change the
!in_task to in_atomic.

I need to do some research on the subtle differences between in_task,
in_atomic, etc.  TBH, I 'thought' !in_task would prevent the issue
reported here.  But, that obviously is not the case.
-- 
Mike Kravetz