linux-kernel - Re: [PATCH v3 1/8] locking/lockdep: Decrement irq context counters when removing lock chain

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Date:   Thu, 16 Jan 2020 10:50:01 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>
Subject: Re: [PATCH v3 1/8] locking/lockdep: Decrement irq context counters
 when removing lock chain

On 1/16/20 10:32 AM, Peter Zijlstra wrote:
> On Wed, Jan 15, 2020 at 04:43:06PM -0500, Waiman Long wrote:
>> There are currently three counters to track the irq context of a lock
>> chain - nr_hardirq_chains, nr_softirq_chains and nr_process_chains.
>> They are incremented when a new lock chain is added, but they are
>> not decremented when a lock chain is removed. That causes some of the
>> statistic counts reported by /proc/lockdep_stats to be incorrect.
>>
>> Fix that by decrementing the right counter when a lock chain is removed.
>>
>> Fixes: a0b0fd53e1e6 ("locking/lockdep: Free lock classes that are no longer in use")
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>  kernel/locking/lockdep.c           | 36 +++++++++++++++++++++---------
>>  kernel/locking/lockdep_internals.h |  6 +++++
>>  2 files changed, 32 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
>> index 32282e7112d3..b20fa6236b2a 100644
>> --- a/kernel/locking/lockdep.c
>> +++ b/kernel/locking/lockdep.c
>> @@ -2299,16 +2299,24 @@ static int check_irq_usage(struct task_struct *curr, struct held_lock *prev,
>>  	return 0;
>>  }
>>  
>> -static void inc_chains(void)
>> +static void inc_chains(int irq_context)
>>  {
>> -	if (current->hardirq_context)
>> +	if (irq_context & LOCK_CHAIN_HARDIRQ_CONTEXT)
>>  		nr_hardirq_chains++;
>> -	else {
>> -		if (current->softirq_context)
>> -			nr_softirq_chains++;
>> -		else
>> -			nr_process_chains++;
>> -	}
>> +	else if (irq_context & LOCK_CHAIN_SOFTIRQ_CONTEXT)
>> +		nr_softirq_chains++;
>> +	else
>> +		nr_process_chains++;
>> +}
>> +
>> +static void dec_chains(int irq_context)
>> +{
>> +	if (irq_context & LOCK_CHAIN_HARDIRQ_CONTEXT)
>> +		nr_hardirq_chains--;
>> +	else if (irq_context & LOCK_CHAIN_SOFTIRQ_CONTEXT)
>> +		nr_softirq_chains--;
>> +	else
>> +		nr_process_chains--;
>>  }
>>  
>>  #else
>> @@ -2324,6 +2332,10 @@ static inline void inc_chains(void)
>>  	nr_process_chains++;
>>  }
>>  
>> +static void dec_chains(int irq_context)
>> +{
>> +	nr_process_chains--;
>> +}
>>  #endif /* CONFIG_TRACE_IRQFLAGS */
>>  
> Is there really need for two versions of those functions? Would the
> @irq_context argument not always be 0 in the CONFIG_TRACE_IRQFLAGS=n
> case?
>
You are right. I changed the code so that inc_chains() won't look at
{hard|soft}irq_context directly. So I could take it out of
CONFIG_TRACE_IRQFLAGS as a single version.

I will make the change in the next version.

Cheers,
Longman