linux-kernel - Re: [PATCH v2 5/8] sched,x86: Enable Turbo Boost Max Technology

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Message-ID: <20160908075955.GS10153@twins.programming.kicks-ass.net>
Date:   Thu, 8 Sep 2016 09:59:55 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Cc:     rjw@...ysocki.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...e.de, x86@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        tim.c.chen@...ux.intel.com
Subject: Re: [PATCH v2 5/8] sched,x86: Enable Turbo Boost Max Technology

On Thu, Sep 01, 2016 at 01:33:41PM -0700, Srinivas Pandruvada wrote:
> +static void enable_sched_itmt(bool enable_itmt)
> +{
> +	mutex_lock(&itmt_update_mutex);
> +
> +	sysctl_sched_itmt_enabled = enable_itmt;
> +	x86_topology_update = true;
> +	rebuild_sched_domains();
> +
> +	mutex_unlock(&itmt_update_mutex);
> +}
> +
> +static int sched_itmt_update_handler(struct ctl_table *table, int write,
> +			      void __user *buffer, size_t *lenp, loff_t *ppos)
> +{
> +	int ret;
> +
> +	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> +
> +	if (ret || !write)
> +		return ret;
> +
> +	enable_sched_itmt(sysctl_sched_itmt_enabled);
> +
> +	return ret;
> +}

I think there's a race here, if two tasks were to write to the sysctl
they'd both change the value before getting stuck on the mutex in
enable_sched_itmt().

One way around that is doing something like:


	struct ctl_table t;
	int val = sysctl_sched_itmt_enabled;

	t = *table;
	t.data = &val;

	proc_dointvec_minmax(&t, ...);

	/* and update the sysctl_sched_itmt_enabled value inside the mutex */
	enable_sched_itmi(val);