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

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Message-ID: <20160311124523.GB6344@twins.programming.kicks-ass.net>
Date:	Fri, 11 Mar 2016 13:45:23 +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 01:43:53PM +0100, Juergen Gross wrote:
> On 11/03/16 13:19, Peter Zijlstra wrote:
> > 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?
> 
> Aah, okay. Seems I should add preempt_disable() in this patch already
> and retry the set_cpus_allowed_ptr() in case cpus_allowed has changed.

Doesn't help, you should simply not _ever_ touch this for random tasks.