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

[Peter Zijlstra]

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);