linux-kernel - [PATCH 1/3] powerpc-enable-preemption-before-cpu

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Date:	Wed,  4 Aug 2010 21:28:33 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	linuxppc-dev@...abs.org, will_schmidt@...t.ibm.com,
	tglx@...utronix.de, nfont@...tin.ibm.com, rcj@...ux.vnet.ibm.com,
	brking@...ux.vnet.ibm.com, dvhltc@...ibm.com
Subject: [PATCH 1/3] powerpc-enable-preemption-before-cpu_die

From: Signed-off-by: Darren Hart <dvhltc@...ibm.com>

start_secondary() is called shortly after _start and also via

cpu_idle()->cpu_die()->pseries_mach_cpu_die()

start_secondary() expects a preempt_count() of 0. pseries_mach_cpu_die() is
called via the cpu_idle() routine with preemption disabled, resulting in the
following repeating message during rapid cpu offline/online tests
with CONFIG_PREEMPT=y:

BUG: scheduling while atomic: swapper/0/0x00000002
Modules linked in: autofs4 binfmt_misc dm_mirror dm_region_hash dm_log [last unloaded: scsi_wait_scan]
Call Trace:
[c00000010e7079c0] [c0000000000133ec] .show_stack+0xd8/0x218 (unreliable)
[c00000010e707aa0] [c0000000006a47f0] .dump_stack+0x28/0x3c
[c00000010e707b20] [c00000000006e7a4] .__schedule_bug+0x7c/0x9c
[c00000010e707bb0] [c000000000699d9c] .schedule+0x104/0x800
[c00000010e707cd0] [c000000000015b24] .cpu_idle+0x1c4/0x1d8
[c00000010e707d70] [c0000000006aa1b4] .start_secondary+0x398/0x3d4
[c00000010e707e30] [c000000000008278] .start_secondary_resume+0x10/0x14

Move the cpu_die() call inside the existing preemption enabled block of
cpu_idle(). This is safe as the idle task is affined to a single CPU so the
debug_smp_processor_id() tests (from cpu_should_die()) won't trigger as we are
in a "migration disabled" region.

Signed-off-by: Darren Hart <dvhltc@...ibm.com>
Acked-by: Will Schmidt <will_schmidt@...t.ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Nathan Fontenot <nfont@...tin.ibm.com>
Cc: Robert Jennings <rcj@...ux.vnet.ibm.com>
Cc: Brian King <brking@...ux.vnet.ibm.com>
---
 arch/powerpc/kernel/idle.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/arch/powerpc/kernel/idle.c b/arch/powerpc/kernel/idle.c
index 049dda6..39a2baa 100644
--- a/arch/powerpc/kernel/idle.c
+++ b/arch/powerpc/kernel/idle.c
@@ -94,9 +94,9 @@ void cpu_idle(void)
 		HMT_medium();
 		ppc64_runlatch_on();
 		tick_nohz_restart_sched_tick();
+		preempt_enable_no_resched();
 		if (cpu_should_die())
 			cpu_die();
-		preempt_enable_no_resched();
 		schedule();
 		preempt_disable();
 	}
-- 
1.7.0.4

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