linux-kernel - [PATCH] sched/core: really pretend early bootup to be a normal task

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Date:	Tue, 05 Jul 2016 14:23:23 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Cc:	Sasha Levin <sasha.levin@...cle.com>,
	Yao Dongdong <yaodongdong@...wei.com>
Subject: [PATCH] sched/core: really pretend early bootup to be a normal task

Commit 1b537c7d1e58 ("sched/core: Remove check of p->sched_class") placed
"current->sched_class = &fair_sched_class" before call of init_idle() which
immediately set sched_class back to idle_sched_class.

This patch reverts this piece because after commit de9b8f5dcbd9 ("sched:
Fix crash trying to dequeue/enqueue the idle thread") init_idle() doesn't
call do_set_cpus_allowed() and nobody will dereference ->sched_class.

[ Probably this pretending is no longer required and could be removed,
  or maybe it helps in catching bugs like sleeping during early bootup? ]

Signed-off-by: Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
Fixes: 1b537c7d1e58 ("sched/core: Remove check of p->sched_class")
Cc: Yao Dongdong <yaodongdong@...wei.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: Sasha Levin <sasha.levin@...cle.com>
---
 kernel/sched/core.c |   10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 51d7105f529a..c23aee77d10d 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7481,11 +7481,6 @@ void __init sched_init(void)
 	enter_lazy_tlb(&init_mm, current);
 
 	/*
-	 * During early bootup we pretend to be a normal task:
-	 */
-	current->sched_class = &fair_sched_class;
-
-	/*
 	 * Make us the idle thread. Technically, schedule() should not be
 	 * called from this thread, however somewhere below it might be,
 	 * but because we are the idle thread, we just pick up running again
@@ -7495,6 +7490,11 @@ void __init sched_init(void)
 
 	calc_load_update = jiffies + LOAD_FREQ;
 
+	/*
+	 * During early bootup we pretend to be a normal task:
+	 */
+	current->sched_class = &fair_sched_class;
+
 #ifdef CONFIG_SMP
 	zalloc_cpumask_var(&sched_domains_tmpmask, GFP_NOWAIT);
 	/* May be allocated at isolcpus cmdline parse time */