linux-kernel - Re: [PATCH 1/3] make migrate disable/enable conditioned on softirq

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Date:	Sun, 15 Dec 2013 15:46:39 +0100
From:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Nicholas Mc Guire <der.herr@...r.at>,
	linux-rt-users@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Andreas Platschek <platschek@....tuwien.ac.at>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] make migrate disable/enable conditioned on
 softirq_nestcnt transition

* Steven Rostedt | 2013-12-05 19:45:30 [-0500]:

>On Fri, 6 Dec 2013 00:42:22 +0100
>Nicholas Mc Guire <der.herr@...r.at> wrote:
>> --- a/kernel/softirq.c
>> +++ b/kernel/softirq.c
>> @@ -569,8 +569,8 @@ static void do_current_softirqs(int need_rcu_bh_qs)
>>  
>>  void local_bh_disable(void)
>>  {
>> -	migrate_disable();
>> -	current->softirq_nestcnt++;
>> +	if (++current->softirq_nestcnt == 1)
>> +		migrate_disable();
>>  }
>>  EXPORT_SYMBOL(local_bh_disable);
>>  
>> @@ -584,8 +584,8 @@ void local_bh_enable(void)
>>  		do_current_softirqs(1);
>>  	local_irq_enable();
>>  
>> -	current->softirq_nestcnt--;
>> -	migrate_enable();
>> +	if (--current->softirq_nestcnt == 0)
>> +		migrate_enable();
>
>I wonder if we should add a:
>
>	BUG_ON(current->softirq_nestcnt < 0);

We have a WARN_ON() in each enable path. That one in local_bh_enable()
isn't part of the context here. If you want to s/WARN_/BUG_/ then I
would prefer not to since there are a few people with no UART and this
could break the system while the current sollution keeps the system
running.

>As for the patch, I haven't found anything wrong with it.
>
>Reviewed-by: Steven Rostedt <rostedt@...dmis.org>
Thanks.

Sebastian
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