processor: get rid of cpu_relax_yield

stop_machine is the only user left of cpu_relax_yield. Given that it
now has special semantics which are tied to stop_machine introduce a
weak stop_machine_yield function which architectures can override, and
get rid of the generic cpu_relax_yield implementation.
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

arch/s390/include/asm/processor.h

@@ -222,12 +222,6 @@ static __no_kasan_or_inline unsigned short stap(void)
 	return cpu_address;
 }
 
-/*
- * Give up the time slice of the virtual PU.
- */
-#define cpu_relax_yield cpu_relax_yield
-void cpu_relax_yield(const struct cpumask *cpumask);
-
 #define cpu_relax() barrier()
 
 #define ECAG_CACHE_ATTRIBUTE	0

arch/s390/kernel/processor.c

@@ -7,6 +7,7 @@
 #define KMSG_COMPONENT "cpu"
 #define pr_fmt(fmt) KMSG_COMPONENT ": " fmt
 
+#include <linux/stop_machine.h>
 #include <linux/cpufeature.h>
 #include <linux/bitops.h>
 #include <linux/kernel.h>
@@ -59,7 +60,7 @@ void s390_update_cpu_mhz(void)
 		on_each_cpu(update_cpu_mhz, NULL, 0);
 }
 
-void notrace cpu_relax_yield(const struct cpumask *cpumask)
+void notrace stop_machine_yield(const struct cpumask *cpumask)
 {
 	int cpu, this_cpu;
 
@@ -73,7 +74,6 @@ void notrace cpu_relax_yield(const struct cpumask *cpumask)
 			smp_yield_cpu(cpu);
 	}
 }
-EXPORT_SYMBOL(cpu_relax_yield);
 
 /*
  * cpu_init - initializes state that is per-CPU.

include/linux/sched.h

@@ -1518,10 +1518,6 @@ static inline int set_cpus_allowed_ptr(struct task_struct *p, const struct cpuma
 }
 #endif
 
-#ifndef cpu_relax_yield
-#define cpu_relax_yield(cpumask) cpu_relax()
-#endif
-
 extern int yield_to(struct task_struct *p, bool preempt);
 extern void set_user_nice(struct task_struct *p, long nice);
 extern int task_prio(const struct task_struct *p);

include/linux/stop_machine.h

@@ -36,6 +36,7 @@ int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
 void stop_machine_park(int cpu);
 void stop_machine_unpark(int cpu);
+void stop_machine_yield(const struct cpumask *cpumask);
 
 #else	/* CONFIG_SMP */
 

kernel/stop_machine.c

@@ -177,6 +177,11 @@ static void ack_state(struct multi_stop_data *msdata)
 		set_state(msdata, msdata->state + 1);
 }
 
+void __weak stop_machine_yield(const struct cpumask *cpumask)
+{
+	cpu_relax();
+}
+
 /* This is the cpu_stop function which stops the CPU. */
 static int multi_cpu_stop(void *data)
 {
@@ -204,7 +209,7 @@ static int multi_cpu_stop(void *data)
 	/* Simple state machine */
 	do {
 		/* Chill out and ensure we re-read multi_stop_state. */
-		cpu_relax_yield(cpumask);
+		stop_machine_yield(cpumask);
 		if (msdata->state != curstate) {
 			curstate = msdata->state;
 			switch (curstate) {