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SubjectRe: [PATCH -tip v2] sched: more sched_domain iterations fix
On 04/26/2011 05:27 PM, Peter Zijlstra wrote:
> On Fri, 2011-04-22 at 18:53 +0800, Xiaotian feng wrote:
>> From: Xiaotian Feng<dfeng@redhat.com>
>>
>> sched_domain iterations needs to be protected by rcu_read_lock() now,
>> this patch adds another two places which needs the rcu lock, which is
>> spotted by following suspicious rcu_dereference_check() usage warnings.
>>
>> kernel/sched_rt.c:1244 invoked rcu_dereference_check() without protection!
>> kernel/sched_stats.h:41 invoked rcu_dereference_check() without protection!
>
> Much better, one worry:
>
>> Signed-off-by: Xiaotian Feng<dfeng@redhat.com>
>> Cc: Ingo Molnar<mingo@elte.hu>
>> Cc: Peter Zijlstra<peterz@infradead.org>
>> ---
>
>> diff --git a/kernel/sched_stats.h b/kernel/sched_stats.h
>> index 48ddf43..331e01b 100644
>> --- a/kernel/sched_stats.h
>> +++ b/kernel/sched_stats.h
>> @@ -37,7 +37,7 @@ static int show_schedstat(struct seq_file *seq, void *v)
>>
>> #ifdef CONFIG_SMP
>> /* domain-specific stats */
>> - preempt_disable();
>> + rcu_read_lock();
>> for_each_domain(cpu, sd) {
>> enum cpu_idle_type itype;
>>
>> @@ -64,7 +64,7 @@ static int show_schedstat(struct seq_file *seq, void *v)
>> sd->ttwu_wake_remote, sd->ttwu_move_affine,
>> sd->ttwu_move_balance);
>> }
>> - preempt_enable();
>> + rcu_read_unlock();
>> #endif
>> }
>> kfree(mask_str);
>
> Did you indeed validate that the preempt_disable() wasn't needed for
> anything else? Your changelog doesn't mention and I didn't check, just
> noticed the possibility on the first posting.
>
Sorry, I just checked them, preempt_disable/enable was introduced by
commit 674311d,
the rcu_read_lock_sched is not existed at that time.

btw, as for_each_domain is protected by rcu_read_lock() and
preempt_disable is not suffice
any more, could we also update comments in for_each_domain?

/*
* The domain tree (rq->sd) is protected by RCU's quiescent state
transition.
* See detach_destroy_domains: synchronize_sched for details.
*
* The domain tree of any CPU may only be accessed from within
* preempt-disabled sections.
*/



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