Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock() like a trylock

[Waiman Long <longman@redhat.com>]

On 3/17/21 10:03 AM, Waiman Long wrote:
> On 3/17/21 9:31 AM, Peter Zijlstra wrote:
>> On Wed, Mar 17, 2021 at 02:12:41PM +0100, Peter Zijlstra wrote:
>>> On Wed, Mar 17, 2021 at 12:38:21PM -0000, tip-bot2 for Waiman Long 
>>> wrote:
>>>> +    /*
>>>> +     * Treat as trylock for ww_mutex.
>>>> +     */
>>>> +    mutex_acquire_nest(&lock->dep_map, subclass, !!ww_ctx, 
>>>> nest_lock, ip);
>>> I'm confused... why isn't nest_lock working here?
>>>
>>> For ww_mutex, we're supposed to have ctx->dep_map as a nest_lock, and
>>> all lock acquisitions under a nest lock should be fine. Afaict the 
>>> above
>>> is just plain wrong.
>> To clarify:
>>
>>     mutex_lock(&A);            ww_mutex_lock(&B, ctx);
>>     ww_mutex_lock(&B, ctx);        mutex_lock(&A);
>>
>> should still very much be a deadlock, but your 'fix' makes it not report
>> that.
>>
>> Only order within the ww_ctx can be ignored, and that's exactly what
>> nest_lock should be doing.
>>
> I will take a deeper look into why that is the case. 

 From reading the source code, nest_lock check is done in 
check_deadlock() so that it won't complain. However, nest_lock isn't 
considered in check_noncircular() which causes the splat to come out. 
Maybe we should add a check for nest_lock there. I will fiddle with the 
code to see if it can address the issue.

Cheers,
Longman