linux-kernel - Re: [PATCH 1/2] sched/topology: Annotate RCU pointers properly

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Message-ID: <04b625e0-58c7-4814-952e-1fb1e1e8ff9c@arm.com>
Date: Thu, 4 Jan 2024 16:46:24 +0100
From: Pierre Gondois <pierre.gondois@....com>
To: Valentin Schneider <vschneid@...hat.com>, linux-kernel@...r.kernel.org
Cc: 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 1/2] sched/topology: Annotate RCU pointers properly

Hello Valentin,

On 1/4/24 10:54, Valentin Schneider wrote:
> On 03/01/24 13:56, Pierre Gondois wrote:
>> 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 ***
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
> 
> That's from when the NUMA topologies were made dynamic, which should be:
> Fixes: 0fb3978b0aac ("sched/numa: Fix NUMA topology for systems with CPU-less nodes")

Ok yes

>> ---
>>   kernel/sched/topology.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
>> index 10d1391e7416..0342a4f41f09 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;
> 
> I understand that's what sparse is asking for, but that looks odd to me. We
> use it as:
> 
>    rcu_assign_pointer(sched_domains_numa_masks, foo);
> 
> so why isn't it
> 
>    __rcu ***sched_domains_numa_masks;
> 
> ?
> 
> This isn't a pointer to an RCU-protected array of masks, this is an
> RCU-protected double array of masks.

I think:
   static struct cpumask ** __rcu *sched_domains_numa_masks;
should denote an RCU-protected array^3 of 'struct cpumask', when
   static struct cpumask __rcu ***sched_domains_numa_masks;
would denote an array^2 of RCU-protected 'struct cpumask*',
and assignments would look like:
   rcu_assign_pointer(**sched_domains_numa_masks, foo);

Meaning that, when taking as a better example:
   static int __rcu *sched_domains_numa_distance;
Here we would like to avoid having 'access after free' to the array of
integer allocated to sched_domains_numa_distance.

For sched_domains_numa_masks:
   static struct cpumask	** __rcu *sched_domains_numa_masks;
   rcu_assign_pointer(sched_domains_numa_masks, foo);
   bar = rcu_dereference(sched_domains_numa_masks);
once the first array pointed by sched_domains_numa_masks is accessed/assigned,
we know the RCU-framework makes it safe against 'accesses after free' when
accessing the level-2 array of sched_domains_numa_masks, or accessing the mask
(level 3).

Please let me know if the reasoning seems dodgy.

About the kernel test robot:
   kernel/sched/topology.c:1998:19: sparse: sparse: incorrect type in assignment (different address spaces) @@
        expected int *distances @@
        got int [noderef] __rcu *static [addressable] [assigned] [toplevel] sched_domains_numa_distance
   kernel/sched/topology.c:2000:15: sparse: sparse: incorrect type in assignment (different address spaces) @@
        expected struct cpumask ***masks @@
        got struct cpumask **[noderef] __rcu *static [addressable] [assigned] [toplevel] sched_domains_numa_masks @@
I think the warnings can be ignored since the two pointers (distances, masks)
are only dereferenced after a 'synchronize_rcu()'

Regards,
Pierre

>>   #endif
>>
>>   /*
>> --
>> 2.25.1
>