Re: [PATCH v6 2/5] sched: Use user_cpus_ptr for saving user provided cpumask in sched_setaffinity()

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Aug 25, 2022 at 09:01:16PM -0400, Waiman Long wrote:


>  void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
>  {
> -	struct cpumask *user_mask = p->user_cpus_ptr;
> -	unsigned long flags;
> -
>  	/*
> -	 * Try to restore the old affinity mask. If this fails, then
> -	 * we free the mask explicitly to avoid it being inherited across
> -	 * a subsequent fork().
> +	 * Try to restore the old affinity mask with __sched_setaffinity().
> +	 * Cpuset masking will be done there too.
>  	 */
> -	if (!user_mask || !__sched_setaffinity(p, user_mask))
> -		return;
> -
> -	raw_spin_lock_irqsave(&p->pi_lock, flags);
> -	user_mask = clear_user_cpus_ptr(p);
> -	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> -
> -	kfree(user_mask);
> +	__sched_setaffinity(p, task_user_cpus(p), false);
>  }

We have an issue with __sched_setaffinity() failing here. I'm not sure
ignoring the failure is the right thing -- but I'm also not enturely
sure what is.

>  void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
> @@ -8081,10 +8046,11 @@ int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask)
>  #endif
>  
>  static int
> -__sched_setaffinity(struct task_struct *p, const struct cpumask *mask)
> +__sched_setaffinity(struct task_struct *p, const struct cpumask *mask, bool save_mask)
>  {
>  	int retval;
>  	cpumask_var_t cpus_allowed, new_mask;
> +	struct cpumask *user_mask = NULL;
>  
>  	if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL))
>  		return -ENOMEM;
> @@ -8100,8 +8066,22 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask)
>  	retval = dl_task_check_affinity(p, new_mask);
>  	if (retval)
>  		goto out_free_new_mask;
> +
> +	/*
> +	 * Save the user requested mask internally now and then update
> +	 * user_cpus_ptr later after making sure this call will be
> +	 * successful, i.e. retval == 0.
> +	 */
> +	if (save_mask) {
> +		user_mask = kmalloc(cpumask_size(), GFP_KERNEL);
> +		if (!user_mask) {
> +			retval = -ENOMEM;
> +			goto out_free_new_mask;
> +		}
> +		cpumask_copy(user_mask, mask);
> +	}
>  again:
> -	retval = __set_cpus_allowed_ptr(p, new_mask, SCA_CHECK | SCA_USER);
> +	retval = __set_cpus_allowed_ptr(p, new_mask, SCA_CHECK);
>  	if (retval)
>  		goto out_free_new_mask;
>  
> @@ -8115,7 +8095,16 @@ __sched_setaffinity(struct task_struct *p, const struct cpumask *mask)
>  		goto again;
>  	}
>  
> +	if (save_mask) {
> +		unsigned long flags;
> +
> +		/* Use pi_lock to synchronize changes to user_cpus_ptr */
> +		raw_spin_lock_irqsave(&p->pi_lock, flags);
> +		swap(p->user_cpus_ptr, user_mask);
> +		raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +	}
>  out_free_new_mask:
> +	kfree(user_mask);
>  	free_cpumask_var(new_mask);
>  out_free_cpus_allowed:
>  	free_cpumask_var(cpus_allowed);

I'm confused as to why it's put in this function and not in the one
caller that actually sets the new @save_mask true, here:

> @@ -8158,7 +8147,7 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>  	if (retval)
>  		goto out_put_task;
>  
> -	retval = __sched_setaffinity(p, in_mask);
> +	retval = __sched_setaffinity(p, in_mask, true);
>  out_put_task:
>  	put_task_struct(p);
>  	return retval;