linux-kernel - Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex

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Date:   Wed, 17 Mar 2021 14:32:27 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org,
        Ingo Molnar <mingo@...nel.org>, x86@...nel.org
Subject: Re: [tip: locking/urgent] locking/ww_mutex: Treat ww_mutex_lock()
 like a trylock

On 3/17/21 1:45 PM, Peter Zijlstra wrote:
> On Wed, Mar 17, 2021 at 06:40:27PM +0100, Peter Zijlstra wrote:
>> On Wed, Mar 17, 2021 at 05:48:48PM +0100, Peter Zijlstra wrote:
>>
>>> I think you'll find that if you use ww_mutex_init() it'll all work. Let
>>> me go and zap this patch, and then I'll try and figure out why
>>> DEFINE_WW_MUTEX() is buggered.
>> Moo, I can't get the compiler to do as I want :/
>>
>> The below is so close but doesn't actually compile.. Maybe we should
>> just give up on DEFINE_WW_MUTEX and simply remove it.
>>
>> ---
>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>> index 0cd631a19727..85f50538f26a 100644
>> --- a/include/linux/mutex.h
>> +++ b/include/linux/mutex.h
>> @@ -129,12 +129,15 @@ do {									\
>>   # define __DEP_MAP_MUTEX_INITIALIZER(lockname)
>>   #endif
>>   
>> -#define __MUTEX_INITIALIZER(lockname) \
>> +#define ___MUTEX_INITIALIZER(lockname, depmap) \
>>   		{ .owner = ATOMIC_LONG_INIT(0) \
>>   		, .wait_lock = __SPIN_LOCK_UNLOCKED(lockname.wait_lock) \
>>   		, .wait_list = LIST_HEAD_INIT(lockname.wait_list) \
>>   		__DEBUG_MUTEX_INITIALIZER(lockname) \
>> -		__DEP_MAP_MUTEX_INITIALIZER(lockname) }
>> +		depmap }
>> +
>> +#define __MUTEX_INITIALIZER(lockname) \
>> +		___MUTEX_INITIALIZER(lockname, __DEP_MAP_MUTEX_INITIALIZER(lockname))
>>   
>>   #define DEFINE_MUTEX(mutexname) \
>>   	struct mutex mutexname = __MUTEX_INITIALIZER(mutexname)
>> diff --git a/include/linux/ww_mutex.h b/include/linux/ww_mutex.h
>> index 6ecf2a0220db..c62a030652b4 100644
>> --- a/include/linux/ww_mutex.h
>> +++ b/include/linux/ww_mutex.h
>> @@ -50,9 +50,17 @@ struct ww_acquire_ctx {
>>   
>>   #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>   # define __WW_CLASS_MUTEX_INITIALIZER(lockname, class) \
>> -		, .ww_class = class
>> +		, .ww_class = &(class)
>> +
>> +# define __DEP_MAP_WW_MUTEX_INITIALIZER(lockname, class) \
>> +		, .dep_map = { \
>> +			.key = &(class).mutex_key, \
>> +			.name = (class).mutex_name, \
> 			,name = #class "_mutex", \
>
> and it 'works', but shees!

The name string itself may be duplicated for multiple instances of 
DEFINE_WW_MUTEX(). Do you want to keep DEFINE_WW_MUTEX() or just use 
ww_mutex_init() for all?

I notice that the problem with DEFINE_WW_MUTEX is that the ww_mutex 
themselves has null key instead of the same key from the ww_class as 
with ww_mutex_init().

Cheers,
Longman