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Date: Fri, 11 Mar 2016 13:19:50 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Juergen Gross <jgross@suse.com>
Cc: linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com,
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        pali.rohar@gmail.com, jdelvare@suse.com, linux@roeck-us.net,
        tglx@linutronix.de, hpa@zytor.com, x86@kernel.org
Subject: Re: [PATCH 2/6] sched: add function to execute a function
 synchronously on a physical cpu
Message-ID: <20160311121950.GZ6344@twins.programming.kicks-ass.net>
References: <1457697574-6710-1-git-send-email-jgross@suse.com>
 <1457697574-6710-3-git-send-email-jgross@suse.com>
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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.