linux-kernel - [tip:sched/core] sched/clock: Fix early boot preempt assumption in __set_sched_clock

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Message-ID: <tip-45aea321678856687927c53972321ebfab77759a@git.kernel.org>
Date:   Wed, 24 May 2017 03:18:56 -0700
From:   tip-bot for Peter Zijlstra <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     torvalds@...ux-foundation.org, mark.rutland@....com,
        xiaolong.ye@...el.com, hpa@...or.com, rostedt@...dmis.org,
        tglx@...utronix.de, gregkh@...uxfoundation.org, mingo@...nel.org,
        linux-kernel@...r.kernel.org, peterz@...radead.org
Subject: [tip:sched/core] sched/clock: Fix early boot preempt assumption in
 __set_sched_clock_stable()

Commit-ID:  45aea321678856687927c53972321ebfab77759a
Gitweb:     http://git.kernel.org/tip/45aea321678856687927c53972321ebfab77759a
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Wed, 24 May 2017 08:52:02 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 24 May 2017 09:10:00 +0200

sched/clock: Fix early boot preempt assumption in __set_sched_clock_stable()

The more strict early boot preemption warnings found that
__set_sched_clock_stable() was incorrectly assuming we'd still be
running on a single CPU:

  BUG: using smp_processor_id() in preemptible [00000000] code: swapper/0/1
  caller is debug_smp_processor_id+0x1c/0x1e
  CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.12.0-rc2-00108-g1c3c5ea #1
  Call Trace:
   dump_stack+0x110/0x192
   check_preemption_disabled+0x10c/0x128
   ? set_debug_rodata+0x25/0x25
   debug_smp_processor_id+0x1c/0x1e
   sched_clock_init_late+0x27/0x87
  [...]

Fix it by disabling IRQs.

Reported-by: kernel test robot <xiaolong.ye@...el.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Acked-by: Thomas Gleixner <tglx@...utronix.de>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: lkp@...org
Cc: tipbuild@...or.com
Link: http://lkml.kernel.org/r/20170524065202.v25vyu7pvba5mhpd@hirez.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/clock.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/clock.c b/kernel/sched/clock.c
index 1a0d389..ca0f8fc 100644
--- a/kernel/sched/clock.c
+++ b/kernel/sched/clock.c
@@ -133,12 +133,19 @@ static void __scd_stamp(struct sched_clock_data *scd)
 
 static void __set_sched_clock_stable(void)
 {
-	struct sched_clock_data *scd = this_scd();
+	struct sched_clock_data *scd;
 
 	/*
+	 * Since we're still unstable and the tick is already running, we have
+	 * to disable IRQs in order to get a consistent scd->tick* reading.
+	 */
+	local_irq_disable();
+	scd = this_scd();
+	/*
 	 * Attempt to make the (initial) unstable->stable transition continuous.
 	 */
 	__sched_clock_offset = (scd->tick_gtod + __gtod_offset) - (scd->tick_raw);
+	local_irq_enable();
 
 	printk(KERN_INFO "sched_clock: Marking stable (%lld, %lld)->(%lld, %lld)\n",
 			scd->tick_gtod, __gtod_offset,