Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed()

[Waiman Long <longman@redhat.com>]

On 11/22/22 14:24, Peter Zijlstra wrote:
> On Tue, Nov 22, 2022 at 10:23:43AM -0500, Waiman Long wrote:
>> index 78b2d5cabcc5..5fac4aa6ac7f 100644
>> --- a/kernel/sched/core.c
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 78b2d5cabcc5..5fac4aa6ac7f 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2593,6 +2593,11 @@ __do_set_cpus_allowed(struct task_struct *p, struct
>> affinity_context *ctx)
>>                  set_next_task(rq, p);
>>   }
>>
>> +union cpumask_rcuhead {
>> +       void *cpumask;
>> +       struct rcu_head rcu;
>> +};
>> +
> Hehe; I had this union too; I just figured it'd be nice to not have to
> spend these 4 lines to express this. Esp. since we're casting pointers
> *anyway*.
Well, that is true. As long as the NULL check is there, I am OK with 
calling kvfree_call_rcu() directly if Paul doesn't object.
>>   /*
>>    * Used for kthread_bind() and select_fallback_rq(), in both cases the user
>>    * affinity (if any) should be destroyed too.
>> @@ -2606,7 +2611,12 @@ void do_set_cpus_allowed(struct task_struct *p, const
>> struct cpumask *new_mask)
>>          };
>>
>>          __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,
>> @@ -8196,7 +8206,7 @@ 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();
>>
>> @@ -8229,7 +8239,11 @@ 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(union cpumask_rcuhead));
>> +       user_mask = kmalloc(size, GFP_KERNEL);
>>          if (!user_mask) {
>>                  retval = -ENOMEM;
>>                  goto out_put_task;
>>
> We also should fix the allocation in dup_user_cpus_ptr() -- perhaps pull
> the thing into a helper.
>
I have just sent out a new patch to fix that before I saw your email. I 
do forgot to put -tip in the subject line.

Cheers,
Longman