Re: [PATCH-tip] sched: Don't call kfree() in do_set_cpus_allowed()
From: Peter Zijlstra
Date: Tue Nov 22 2022 - 14:24:22 EST
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.