linux-kernel - Re: [PATCH] sched: rt_rq runtime leakage bug fix

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Message-ID: <4DCBBC9D.7020908@linux.vnet.ibm.com>
Date:	Thu, 12 May 2011 18:55:25 +0800
From:	Cheng Xu <chengxu@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Ingo Molnar <mingo@...e.hu>,
	Paul Mckenney <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: rt_rq runtime leakage bug fix

On 5/12/2011 18:12, Peter Zijlstra wrote:
> On Thu, 2011-05-12 at 01:30 +0800, Cheng Xu wrote:
>>
>> I tried but hit a boot-time error "Unable to handle kernel paging
>> request for data at address 0x100000008", and therefore would like to
>> propose an alternative patch like,
>>
> I probably made a silly mistake somehwere, it was after all something
> quickly typed in an email :-)
> 
>> #define for_each_rt_rq(rt_rq, iter, rq) \
>>         for (iter = list_entry_rcu(task_groups.next, typeof(*iter), list); \
>>              (&iter->list != &task_groups) && (rt_rq = iter->rt_rq[cpu_of(rq)]); \
>>              iter = list_entry_rcu(iter->list.next, typeof(*iter), list))
>>
>> This worked, it seems to pass the tests.  Is this correct from a scheduler perspective?
> 
> Creative ;-), it would be nice to know why the , operator version
> doesn't work though, since that looks to be the more conventional way to
> write it.

Yes I am also wondering why it doesn't work. will look into it and get
back to you later.

> 
> That said, I don't see a problem with using your existing on.
> 
>> For the not CONFIG_RT_GROUP_SCHED part, I used 
>>
>> typedef struct rt_rq *rt_rq_iter_t;
>>
>> #define for_each_rt_rq(rt_rq, iter, rq) \
>>         (void) iter; \
>>         for (rt_rq = &rq->rt; rt_rq; rt_rq = NULL)
>>
>> An alternative is 
>> #define for_each_rt_rq(rt_rq, iter, rq) \
>>         for (rt_rq = iter = &rq->rt; iter; rt_rq = iter = NULL)
> 
> Tough call that, the first has a multi-statement macro, which is
> generally discouraged because then:
> 
>   for()
>    for_each_rt_rq() {
>    }
> 
> will not work as expected, so I think we want the second version.

Agree, I realized this problem soon after sending out the email
yesterday, :) and improved it to be

#define for_each_rt_rq(rt_rq, iter, rq) \
	for ((void) iter, rt_rq = &rq->rt; rt_rq; rt_rq = NULL)

maybe we can still use it?

> 
>> The patch is attached below. Could you check whether it is workable? Thank you. 
> 
> Yes, given how things are I can't really see it getting any better,
> thanks!
> 

I have updated the patch content according to the comments, and done
part of the test. will send out the complete second version for your
review soon.

Thank you very much!

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