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

[Peter Zijlstra]

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*.

>  /*
>   * 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.