Re: [PATCH-tip v2] sched: Use kfree_rcu() in do_set_cpus_allowed()

[Waiman Long <longman@redhat.com>]

On 11/28/22 20:26, 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@infradead.org>
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>   kernel/sched/core.c | 21 ++++++++++++++++++---
>   1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f7c5253bfd21..bf9066b4f481 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;
> +};
> +
>   /*
>    * Used for kthread_bind() and select_fallback_rq(), in both cases the user
>    * affinity (if any) should be destroyed too.
> @@ -2606,7 +2611,13 @@ 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,
> @@ -8220,7 +8231,7 @@ 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 +8264,11 @@ 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;

Peter, I would like to get this done before the next merge window. I 
personally prefer to define a useless union to facilitate the use of 
kfree_rcu(). I can also live without it and call kvfree_call_rcu() 
directly if this is what you want.

Cheers,
Longman