cpufreq/sparc-us3: Replace racy task affinity logic

The access to the safari config register in the CPU frequency functions
must be executed on the target CPU. This is achieved by temporarily setting
the affinity of the calling user space thread to the requested CPU and
reset it to the original affinity afterwards.

That's racy vs. CPU hotplug and concurrent affinity settings for that
thread resulting in code executing on the wrong CPU and overwriting the
new affinity setting.

Replace it by a straight forward smp function call. 
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Sebastian Siewior <bigeasy@linutronix.de>
Cc: linux-pm@vger.kernel.org
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Len Brown <lenb@kernel.org>
Link: http://lkml.kernel.org/r/20170412201043.047558840@linutronix.deSigned-off-by: Thomas Gleixner <tglx@linutronix.de>

drivers/cpufreq/sparc-us3-cpufreq.c

@@ -35,22 +35,28 @@ static struct us3_freq_percpu_info *us3_freq_table;
 #define SAFARI_CFG_DIV_32	0x0000000080000000UL
 #define SAFARI_CFG_DIV_MASK	0x00000000C0000000UL
 
-static unsigned long read_safari_cfg(void)
+static void read_safari_cfg(void *arg)
 {
-	unsigned long ret;
+	unsigned long ret, *val = arg;
 
 	__asm__ __volatile__("ldxa	[%%g0] %1, %0"
 			     : "=&r" (ret)
 			     : "i" (ASI_SAFARI_CONFIG));
-	return ret;
+	*val = ret;
 }
 
-static void write_safari_cfg(unsigned long val)
+static void update_safari_cfg(void *arg)
 {
+	unsigned long reg, *new_bits = arg;
+
+	read_safari_cfg(&reg);
+	reg &= ~SAFARI_CFG_DIV_MASK;
+	reg |= *new_bits;
+
 	__asm__ __volatile__("stxa	%0, [%%g0] %1\n\t"
 			     "membar	#Sync"
 			     : /* no outputs */
-			     : "r" (val), "i" (ASI_SAFARI_CONFIG)
+			     : "r" (reg), "i" (ASI_SAFARI_CONFIG)
 			     : "memory");
 }
 
@@ -78,29 +84,17 @@ static unsigned long get_current_freq(unsigned int cpu, unsigned long safari_cfg
 
 static unsigned int us3_freq_get(unsigned int cpu)
 {
-	cpumask_t cpus_allowed;
 	unsigned long reg;
-	unsigned int ret;
-
-	cpumask_copy(&cpus_allowed, &current->cpus_allowed);
-	set_cpus_allowed_ptr(current, cpumask_of(cpu));
-
-	reg = read_safari_cfg();
-	ret = get_current_freq(cpu, reg);
-
-	set_cpus_allowed_ptr(current, &cpus_allowed);
 
-	return ret;
+	if (smp_call_function_single(cpu, read_safari_cfg, &reg, 1))
+		return 0;
+	return get_current_freq(cpu, reg);
 }
 
 static int us3_freq_target(struct cpufreq_policy *policy, unsigned int index)
 {
 	unsigned int cpu = policy->cpu;
-	unsigned long new_bits, new_freq, reg;
-	cpumask_t cpus_allowed;
-
-	cpumask_copy(&cpus_allowed, &current->cpus_allowed);
-	set_cpus_allowed_ptr(current, cpumask_of(cpu));
+	unsigned long new_bits, new_freq;
 
 	new_freq = sparc64_get_clock_tick(cpu) / 1000;
 	switch (index) {
@@ -121,15 +115,7 @@ static int us3_freq_target(struct cpufreq_policy *policy, unsigned int index)
 		BUG();
 	}
 
-	reg = read_safari_cfg();
-
-	reg &= ~SAFARI_CFG_DIV_MASK;
-	reg |= new_bits;
-	write_safari_cfg(reg);
-
-	set_cpus_allowed_ptr(current, &cpus_allowed);
-
-	return 0;
+	return smp_call_function_single(cpu, update_safari_cfg, &new_bits, 1);
 }
 
 static int __init us3_freq_cpu_init(struct cpufreq_policy *policy)