Re: [PATCH-tip v3] sched: Use kfree_rcu() in do_set_cpus_allowed()

[Peter Zijlstra]

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@xxxxxxxxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
>  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().