linux-kernel - Re: [PATCH v2 1/3] sched/topology: Annotate RCU pointers properly

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Date: Fri, 02 Feb 2024 18:35:35 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: "Huang, Ying" <ying.huang@...el.com>, Pierre Gondois
 <pierre.gondois@....com>
Cc: linux-kernel@...r.kernel.org, Shrikanth Hegde
 <sshegde@...ux.vnet.ibm.com>, Ingo Molnar <mingo@...hat.com>, Peter
 Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
 <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben
 Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Daniel
 Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [PATCH v2 1/3] sched/topology: Annotate RCU pointers properly

On 16/01/24 09:23, Huang, Ying wrote:
> Pierre Gondois <pierre.gondois@....com> writes:
>
>> Cleanup RCU-related spare errors by annotating RCU pointers.
>>
>> sched_domains_numa_distance:
>>   error: incompatible types in comparison expression
>>   (different address spaces):
>>       int [noderef] __rcu *
>>       int *
>>
>> sched_domains_numa_masks:
>>   error: incompatible types in comparison expression
>>   (different address spaces):
>>       struct cpumask **[noderef] __rcu *
>>       struct cpumask ***
>>
>> The cast to (void *) adds the following sparse warning:
>>   warning: cast removes address space '__rcu' of expression
>> but this should be normal.
>>
>> Fixes: 0fb3978b0aac ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")
>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
>> ---
>>  kernel/sched/topology.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 10d1391e7416..2a2da9b33e31 100644
>> --- a/kernel/sched/topology.c
>> +++ b/kernel/sched/topology.c
>> @@ -1542,8 +1542,8 @@ static int			sched_domains_numa_levels;
>>  static int			sched_domains_curr_level;
>>
>>  int				sched_max_numa_distance;
>> -static int			*sched_domains_numa_distance;
>> -static struct cpumask		***sched_domains_numa_masks;
>> +static int			__rcu *sched_domains_numa_distance;
>> +static struct cpumask		** __rcu *sched_domains_numa_masks;
>>  #endif
>>
>>  /*
>> @@ -1988,8 +1988,8 @@ void sched_init_numa(int offline_node)
>>
>>  static void sched_reset_numa(void)
>>  {
>> -	int nr_levels, *distances;
>> -	struct cpumask ***masks;
>> +	int nr_levels, __rcu *distances;
>> +	struct cpumask ** __rcu *masks;
>
> No.  distances and masks are not accessed via RCU in the function.
> Instead, they should be assigned like below,
>
>          distances = rcu_dereference_raw(sched_domains_numa_distance);
>
> Because sched_domains_numa_distance is protected by cpu_hotplug_lock,
> but the lock is static.  Some comments are deserved.
>
> Anyway, please read RCU document before making the change.
>

IIUC, something like so could also do?

  distances = rcu_dereference_check(sched_domains_numa_distance, lockdep_is_cpus_held());