linux-kernel - Re: [PATCH 2/6] sched: add function to execute a function synchronously on a physical cpu

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Date:	Fri, 11 Mar 2016 13:19:50 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juergen Gross <jgross@...e.com>
Cc:	linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
	konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
	david.vrabel@...rix.com, mingo@...hat.com,
	Douglas_Warzecha@...l.com, pali.rohar@...il.com, jdelvare@...e.com,
	linux@...ck-us.net, tglx@...utronix.de, hpa@...or.com,
	x86@...nel.org
Subject: Re: [PATCH 2/6] sched: add function to execute a function
 synchronously on a physical cpu

On Fri, Mar 11, 2016 at 12:59:30PM +0100, Juergen Gross wrote:
> +int call_sync_on_phys_cpu(unsigned cpu, int (*func)(void *), void *par)
> +{
> +	cpumask_var_t old_mask;
> +	int ret;
> +
> +	if (cpu >= nr_cpu_ids)
> +		return -EINVAL;
> +
> +	if (!alloc_cpumask_var(&old_mask, GFP_KERNEL))
> +		return -ENOMEM;
> +
> +	cpumask_copy(old_mask, &current->cpus_allowed);
> +	ret = set_cpus_allowed_ptr(current, cpumask_of(cpu));
> +	if (ret)
> +		goto out;

So what happens if someone does sched_setaffinity() right about here?

> +
> +	ret = func(par);
> +
> +	set_cpus_allowed_ptr(current, old_mask);
> +
> +out:
> +	free_cpumask_var(old_mask);
> +	return ret;
> +}
> +EXPORT_SYMBOL_GPL(call_sync_on_phys_cpu);

This is disgusting, and you're adding this to !Xen kernels too.