linux-kernel - Re: [PATCH-tip v3] sched: Use kfree_rcu() in do_set_cpus

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Message-ID: <56792f82-951e-04c5-f7ea-fbf9ab375eec@redhat.com>
Date:   Thu, 22 Dec 2022 15:18:29 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, 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>,
        Phil Auld <pauld@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH-tip v3] sched: Use kfree_rcu() in do_set_cpus_allowed()

On 12/22/22 14:32, Peter Zijlstra wrote:
> On Mon, Dec 05, 2022 at 11:39:36AM -0500, Waiman Long wrote:
>> Commit 851a723e45d1 ("sched: Always clear user_cpus_ptr in
>> do_set_cpus_allowed()") may call kfree() if user_cpus_ptr was previously
>> set. Unfortunately, some of the callers of do_set_cpus_allowed()
>> may have pi_lock held when calling it. So the following splats may be
>> printed especially when running with a PREEMPT_RT kernel:
>>
>>     WARNING: possible circular locking dependency detected
>>     BUG: sleeping function called from invalid context
>>
>> To avoid these problems, kfree_rcu() is used instead. An internal
>> cpumask_rcuhead union is created for the sole purpose of facilitating
>> the use of kfree_rcu() to free the cpumask.
>>
>> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
>> Suggested-by: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Waiman Long <longman@...hat.com>
>> ---
>>   kernel/sched/core.c | 20 +++++++++++++++++---
>>   1 file changed, 17 insertions(+), 3 deletions(-)
>>
>>   [v3: Fix build problem reported by kernel test robot]
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 57e5932f81a9..155b6cfe119a 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2604,9 +2604,19 @@ void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask)
>>   		.user_mask = NULL,
>>   		.flags     = SCA_USER,	/* clear the user requested mask */
>>   	};
>> +	union cpumask_rcuhead {
>> +		cpumask_t cpumask;
>> +		struct rcu_head rcu;
>> +	};
>>   
>>   	__do_set_cpus_allowed(p, &ac);
>> -	kfree(ac.user_mask);
>> +
>> +	/*
>> +	 * Because this is called with p->pi_lock held, it is not possible
>> +	 * to use kfree() here (when PREEMPT_RT=y), therefore punt to using
>> +	 * kfree_rcu().
>> +	 */
>> +	kfree_rcu((union cpumask_rcuhead *)ac.user_mask, rcu);
>>   }
>>   
>>   int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>> @@ -8220,7 +8230,7 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>>   	struct affinity_context ac;
>>   	struct cpumask *user_mask;
>>   	struct task_struct *p;
>> -	int retval;
>> +	int retval, size;
>>   
>>   	rcu_read_lock();
>>   
>> @@ -8253,7 +8263,11 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>>   	if (retval)
>>   		goto out_put_task;
>>   
>> -	user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
>> +	/*
>> +	 * See do_set_cpus_allowed() for the rcu_head usage.
>> +	 */
>> +	size = max_t(int, cpumask_size(), sizeof(struct rcu_head));
>> +	user_mask = kmalloc(size, GFP_KERNEL);
>>   	if (!user_mask) {
>>   		retval = -ENOMEM;
>>   		goto out_put_task;
> AFAICT you forgot dup_user_cpus_ptr().

I haven't received any response from you for a while. So it is just a 
ping. Of course, I am aware that there is another dup_user_cpus_ptr() 
patch ouststanding. I will of course talk about that when you respond. I 
also have a pending rwsem patch series waiting for your review:-)

Cheers,
Longman