sched/clock: Fix hotplug crash

Mike reported that he could trigger the WARN_ON_ONCE() in
set_sched_clock_stable() using hotplug.

This exposed a fundamental problem with the interface, we should never
mark the TSC stable if we ever find it to be unstable. Therefore
set_sched_clock_stable() is a broken interface.

The reason it existed is that not having it is a pain, it means all
relevant architecture code needs to call clear_sched_clock_stable()
where appropriate.

Of the three architectures that select HAVE_UNSTABLE_SCHED_CLOCK ia64
and parisc are trivial in that they never called
set_sched_clock_stable(), so add an unconditional call to
clear_sched_clock_stable() to them.

For x86 the story is a lot more involved, and what this patch tries to
do is ensure we preserve the status quo. So even is Cyrix or Transmeta
have usable TSC they never called set_sched_clock_stable() so they now
get an explicit mark unstable.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 9881b024b7 ("sched/clock: Delay switching sched_clock to stable")
Link: http://lkml.kernel.org/r/20170119133633.GB6536@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/ia64/kernel/setup.c

@@ -619,6 +619,8 @@ setup_arch (char **cmdline_p)
 	check_sal_cache_flush();
 #endif
 	paging_init();
+
+	clear_sched_clock_stable();
 }
 
 /*

arch/parisc/kernel/setup.c

@@ -36,6 +36,7 @@
 #undef PCI_DEBUG
 #include <linux/proc_fs.h>
 #include <linux/export.h>
+#include <linux/sched.h>
 
 #include <asm/processor.h>
 #include <asm/sections.h>
@@ -176,6 +177,7 @@ void __init setup_arch(char **cmdline_p)
 	conswitchp = &dummy_con;	/* we use do_take_over_console() later ! */
 #endif
 
+	clear_sched_clock_stable();
 }
 
 /*

arch/x86/kernel/cpu/amd.c

@@ -548,8 +548,10 @@ static void early_init_amd(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Bit 12 of 8000_0007 edx is accumulated power mechanism. */

arch/x86/kernel/cpu/centaur.c

@@ -1,5 +1,5 @@
-#include <linux/bitops.h>
-#include <linux/kernel.h>
+
+#include <linux/sched.h>
 
 #include <asm/cpufeature.h>
 #include <asm/e820.h>
@@ -104,6 +104,8 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
 #ifdef CONFIG_X86_64
 	set_cpu_cap(c, X86_FEATURE_SYSENTER32);
 #endif
+
+	clear_sched_clock_stable();
 }
 
 static void init_centaur(struct cpuinfo_x86 *c)

arch/x86/kernel/cpu/common.c

@@ -83,6 +83,7 @@ static void default_init(struct cpuinfo_x86 *c)
 			strcpy(c->x86_model_id, "386");
 	}
 #endif
+	clear_sched_clock_stable();
 }
 
 static const struct cpu_dev default_cpu = {
@@ -1055,6 +1056,8 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	 */
 	if (this_cpu->c_init)
 		this_cpu->c_init(c);
+	else
+		clear_sched_clock_stable();
 
 	/* Disable the PN if appropriate */
 	squash_the_stupid_serial_number(c);

arch/x86/kernel/cpu/cyrix.c

@@ -9,6 +9,7 @@
 #include <asm/pci-direct.h>
 #include <asm/tsc.h>
 #include <asm/cpufeature.h>
+#include <linux/sched.h>
 
 #include "cpu.h"
 
@@ -183,6 +184,7 @@ static void early_init_cyrix(struct cpuinfo_x86 *c)
 		set_cpu_cap(c, X86_FEATURE_CYRIX_ARR);
 		break;
 	}
+	clear_sched_clock_stable();
 }
 
 static void init_cyrix(struct cpuinfo_x86 *c)

arch/x86/kernel/cpu/intel.c

@@ -124,8 +124,10 @@ static void early_init_intel(struct cpuinfo_x86 *c)
 	if (c->x86_power & (1 << 8)) {
 		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
 		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
-		if (!check_tsc_unstable())
-			set_sched_clock_stable();
+		if (check_tsc_unstable())
+			clear_sched_clock_stable();
+	} else {
+		clear_sched_clock_stable();
 	}
 
 	/* Penwell and Cloverview have the TSC which doesn't sleep on S3 */

arch/x86/kernel/cpu/transmeta.c

@@ -1,4 +1,5 @@
 #include <linux/kernel.h>
+#include <linux/sched.h>
 #include <linux/mm.h>
 #include <asm/cpufeature.h>
 #include <asm/msr.h>
@@ -14,6 +15,8 @@ static void early_init_transmeta(struct cpuinfo_x86 *c)
 		if (xlvl >= 0x80860001)
 			c->x86_capability[CPUID_8086_0001_EDX] = cpuid_edx(0x80860001);
 	}
+
+	clear_sched_clock_stable();
 }
 
 static void init_transmeta(struct cpuinfo_x86 *c)

arch/x86/kernel/kvmclock.c

@@ -107,12 +107,12 @@ static inline void kvm_sched_clock_init(bool stable)
 {
 	if (!stable) {
 		pv_time_ops.sched_clock = kvm_clock_read;
+		clear_sched_clock_stable();
 		return;
 	}
 
 	kvm_sched_clock_offset = kvm_clock_read();
 	pv_time_ops.sched_clock = kvm_sched_clock_read;
-	set_sched_clock_stable();
 
 	printk(KERN_INFO "kvm-clock: using sched offset of %llu cycles\n",
 			kvm_sched_clock_offset);

include/linux/sched.h

@@ -2547,7 +2547,6 @@ extern void sched_clock_init_late(void);
  * is reliable after all:
  */
 extern int sched_clock_stable(void);
-extern void set_sched_clock_stable(void);
 extern void clear_sched_clock_stable(void);
 
 extern void sched_clock_tick(void);

kernel/sched/clock.c

@@ -83,8 +83,15 @@ void sched_clock_init(void)
 }
 
 #ifdef CONFIG_HAVE_UNSTABLE_SCHED_CLOCK
+/*
+ * We must start with !__sched_clock_stable because the unstable -> stable
+ * transition is accurate, while the stable -> unstable transition is not.
+ *
+ * Similarly we start with __sched_clock_stable_early, thereby assuming we
+ * will become stable, such that there's only a single 1 -> 0 transition.
+ */
 static DEFINE_STATIC_KEY_FALSE(__sched_clock_stable);
-static int __sched_clock_stable_early;
+static int __sched_clock_stable_early = 1;
 
 /*
  * We want: ktime_get_ns() + gtod_offset == sched_clock() + raw_offset
@@ -132,24 +139,6 @@ static void __set_sched_clock_stable(void)
 	tick_dep_clear(TICK_DEP_BIT_CLOCK_UNSTABLE);
 }
 
-void set_sched_clock_stable(void)
-{
-	__sched_clock_stable_early = 1;
-
-	smp_mb(); /* matches sched_clock_init_late() */
-
-	/*
-	 * This really should only be called early (before
-	 * sched_clock_init_late()) when guestimating our sched_clock() is
-	 * solid.
-	 *
-	 * After that we test stability and we can negate our guess using
-	 * clear_sched_clock_stable, possibly from a watchdog.
-	 */
-	if (WARN_ON_ONCE(sched_clock_running == 2))
-		__set_sched_clock_stable();
-}
-
 static void __clear_sched_clock_stable(struct work_struct *work)
 {
 	struct sched_clock_data *scd = this_scd();
@@ -199,8 +188,6 @@ void sched_clock_init_late(void)
 
 	if (__sched_clock_stable_early)
 		__set_sched_clock_stable();
-	else
-		__clear_sched_clock_stable(NULL);
 }
 
 /*