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authorPeter Zijlstra <peterz@infradead.org>2017-01-19 14:36:33 +0100
committerIngo Molnar <mingo@kernel.org>2017-01-20 02:38:46 +0100
commitacb04058de49458010c44bb35b849d45113fd668 (patch)
tree6011ca77b47f21fcd5e65e5a2f02dddadd3f950c /kernel/sched/clock.c
parent02cfdc95a0104fa5812d855d1e4ec687312aaa6f (diff)
downloadlinux-acb04058de49458010c44bb35b849d45113fd668.tar.bz2
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: 9881b024b7d7 ("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>
Diffstat (limited to 'kernel/sched/clock.c')
-rw-r--r--kernel/sched/clock.c29
1 files changed, 8 insertions, 21 deletions
diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 7713b2b53f61..ad64efe41722 100644
--- a/kernel/sched/clock.c
+++ b/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);
}
/*