Commit message: sched/clock: Prevent tracing recursion in sched_clock_cpu()

Prevent tracing of preempt_disable/enable() in sched_clock_cpu().
When CONFIG_DEBUG_PREEMPT is enabled, preempt_disable/enable() are
traced and this causes trace_clock() users (and probably others) to
go into an infinite recursion. Systems with a stable sched_clock()
are not affected.

This problem is similar to that fixed by upstream commit 95ef1e52922
("KVM guest: prevent tracing recursion with kvmclock").

Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1394083528.4524.3.camel@nexus
Signed-off-by: Ingo Molnar <mingo@kernel.org>
CVE: None
Vulnerability description: None

Project: linux
File: clock.c

static inline struct sched_clock_data *cpu_sdc(int cpu)
{
	return &per_cpu(sched_clock_data, cpu);
}

Project: linux
File: clock.c

u64 sched_clock_cpu(int cpu)
{
	struct sched_clock_data *scd;
	u64 clock;

	if (sched_clock_stable())
		return sched_clock();

	if (unlikely(!sched_clock_running))
		return 0ull;

	preempt_disable_notrace();
	scd = cpu_sdc(cpu);

	if (cpu != smp_processor_id())
		clock = sched_clock_remote(scd);
	else
		clock = sched_clock_local(scd);
	preempt_enable_notrace();

	return clock;
}

Project: linux
File: clock.c

void sched_clock_init(void)
{
	sched_clock_running = 1;
}

Project: linux
File: clock.c

void set_sched_clock_stable(void)
{
	__sched_clock_stable_early = 1;

	smp_mb(); /* matches sched_clock_init() */

	if (!sched_clock_running)
		return;

	__set_sched_clock_stable();
}

Project: linux
File: clock.c

unsigned long long __attribute__((weak)) sched_clock(void)
{
	return (unsigned long long)(jiffies - INITIAL_JIFFIES)
					* (NSEC_PER_SEC / HZ);
}

Project: linux
File: clock.c

static inline u64 wrap_min(u64 x, u64 y)
{
	return (s64)(x - y) < 0 ? x : y;
}

Project: linux
File: clock.c

u64 cpu_clock(int cpu)
{
	return sched_clock();
}

Project: linux
File: clock.c

static u64 sched_clock_remote(struct sched_clock_data *scd)
{
	struct sched_clock_data *my_scd = this_scd();
	u64 this_clock, remote_clock;
	u64 *ptr, old_val, val;

#if BITS_PER_LONG != 64
again:
	/*
	 * Careful here: The local and the remote clock values need to
	 * be read out atomic as we need to compare the values and
	 * then update either the local or the remote side. So the
	 * cmpxchg64 below only protects one readout.
	 *
	 * We must reread via sched_clock_local() in the retry case on
	 * 32bit as an NMI could use sched_clock_local() via the
	 * tracer and hit between the readout of
	 * the low32bit and the high 32bit portion.
	 */
	this_clock = sched_clock_local(my_scd);
	/*
	 * We must enforce atomic readout on 32bit, otherwise the
	 * update on the remote cpu can hit inbetween the readout of
	 * the low32bit and the high 32bit portion.
	 */
	remote_clock = cmpxchg64(&scd->clock, 0, 0);
#else
	/*
	 * On 64bit the read of [my]scd->clock is atomic versus the
	 * update, so we can avoid the above 32bit dance.
	 */
	sched_clock_local(my_scd);
again:
	this_clock = my_scd->clock;
	remote_clock = scd->clock;
#endif

	/*
	 * Use the opportunity that we have both locks
	 * taken to couple the two clocks: we take the
	 * larger time as the latest time for both
	 * runqueues. (this creates monotonic movement)
	 */
	if (likely((s64)(remote_clock - this_clock) < 0)) {
		ptr = &scd->clock;
		old_val = remote_clock;
		val = this_clock;
	} else {
		/*
		 * Should be rare, but possible:
		 */
		ptr = &my_scd->clock;
		old_val = this_clock;
		val = remote_clock;
	}

	if (cmpxchg64(ptr, old_val, val) != old_val)
		goto again;

	return val;
}

Project: linux
File: clock.c

void sched_clock_idle_wakeup_event(u64 delta_ns)
{
	if (timekeeping_suspended)
		return;

	sched_clock_tick();
	touch_softlockup_watchdog();
}

Project: linux
File: clock.c

void sched_clock_tick(void)
{
	struct sched_clock_data *scd;
	u64 now, now_gtod;

	if (sched_clock_stable())
		return;

	if (unlikely(!sched_clock_running))
		return;

	WARN_ON_ONCE(!irqs_disabled());

	scd = this_scd();
	now_gtod = ktime_to_ns(ktime_get());
	now = sched_clock();

	scd->tick_raw = now;
	scd->tick_gtod = now_gtod;
	sched_clock_local(scd);
}

Project: linux
File: clock.c

u64 cpu_clock(int cpu)
{
	if (!sched_clock_stable())
		return sched_clock_cpu(cpu);

	return sched_clock();
}

Project: linux
File: clock.c

static u64 sched_clock_local(struct sched_clock_data *scd)
{
	u64 now, clock, old_clock, min_clock, max_clock;
	s64 delta;

again:
	now = sched_clock();
	delta = now - scd->tick_raw;
	if (unlikely(delta < 0))
		delta = 0;

	old_clock = scd->clock;

	/*
	 * scd->clock = clamp(scd->tick_gtod + delta,
	 *		      max(scd->tick_gtod, scd->clock),
	 *		      scd->tick_gtod + TICK_NSEC);
	 */

	clock = scd->tick_gtod + delta;
	min_clock = wrap_max(scd->tick_gtod, old_clock);
	max_clock = wrap_max(old_clock, scd->tick_gtod + TICK_NSEC);

	clock = wrap_max(clock, min_clock);
	clock = wrap_min(clock, max_clock);

	if (cmpxchg64(&scd->clock, old_clock, clock) != old_clock)
		goto again;

	return clock;
}

Project: linux
File: clock.c

static inline u64 wrap_max(u64 x, u64 y)
{
	return (s64)(x - y) > 0 ? x : y;
}

Project: linux
File: clock.c

void sched_clock_idle_sleep_event(void)
{
	sched_clock_cpu(smp_processor_id());
}

Project: linux
File: clock.c

void clear_sched_clock_stable(void)
{
	__sched_clock_stable_early = 0;

	smp_mb(); /* matches sched_clock_init() */

	if (!sched_clock_running)
		return;

	schedule_work(&sched_clock_work);
}

Project: linux
File: clock.c

u64 local_clock(void)
{
	if (!sched_clock_stable())
		return sched_clock_cpu(raw_smp_processor_id());

	return sched_clock();
}

Project: linux
File: clock.c

u64 local_clock(void)
{
	return sched_clock();
}

Project: linux
File: clock.c

static void __clear_sched_clock_stable(struct work_struct *work)
{
	/* XXX worry about clock continuity */
	if (sched_clock_stable())
		static_key_slow_dec(&__sched_clock_stable);
}

Project: linux
File: clock.c

static inline struct sched_clock_data *this_scd(void)
{
	return &__get_cpu_var(sched_clock_data);
}

Project: linux
File: clock.c

static void __set_sched_clock_stable(void)
{
	if (!sched_clock_stable())
		static_key_slow_inc(&__sched_clock_stable);
}

Project: linux
File: clock.c

void sched_clock_init(void)
{
	u64 ktime_now = ktime_to_ns(ktime_get());
	int cpu;

	for_each_possible_cpu(cpu) {
		struct sched_clock_data *scd = cpu_sdc(cpu);

		scd->tick_raw = 0;
		scd->tick_gtod = ktime_now;
		scd->clock = ktime_now;
	}

	sched_clock_running = 1;

	/*
	 * Ensure that it is impossible to not do a static_key update.
	 *
	 * Either {set,clear}_sched_clock_stable() must see sched_clock_running
	 * and do the update, or we must see their __sched_clock_stable_early
	 * and do the update, or both.
	 */
	smp_mb(); /* matches {set,clear}_sched_clock_stable() */

	if (__sched_clock_stable_early)
		__set_sched_clock_stable();
	else
		__clear_sched_clock_stable(NULL);
}

Project: linux
File: clock.c

int sched_clock_stable(void)
{
	return static_key_false(&__sched_clock_stable);
}