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Date:   Fri, 15 Oct 2021 12:39:02 -0500
From:   Nathan Lynch <>
Subject: [PATCH v2] powerpc/smp: do not decrement idle task preempt count in CPU offline

With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we

BUG: scheduling while atomic: swapper/1/0/0x00000000
no locks held by swapper/1/0.
CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.15.0-rc2+ #100
Call Trace:

This is because powerpc's arch_cpu_idle_dead() decrements the idle task's
preempt count, for reasons explained in commit a7c2bb8279d2 ("powerpc:
Re-enable preemption before cpu_die()"), specifically "start_secondary()
expects a preempt_count() of 0."

However, since commit 2c669ef6979c ("powerpc/preempt: Don't touch the idle
task's preempt_count during hotplug") and commit f1a0a376ca0c ("sched/core:
Initialize the idle task with preemption disabled"), that justification no
longer holds.

The idle task isn't supposed to re-enable preemption, so remove the
vestigial preempt_enable() from the CPU offline path.

Tested with pseries and powernv in qemu, and pseries on PowerVM.

Fixes: 2c669ef6979c ("powerpc/preempt: Don't touch the idle task's preempt_count during hotplug")
Signed-off-by: Nathan Lynch <>
Reviewed-by: Valentin Schneider <>

    Changes since v1:
    - remove incorrect Fixes: tag, add Valentin's r-b.

 arch/powerpc/kernel/smp.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
index 9cc7d3dbf439..605bab448f84 100644
--- a/arch/powerpc/kernel/smp.c
+++ b/arch/powerpc/kernel/smp.c
@@ -1730,8 +1730,6 @@ void __cpu_die(unsigned int cpu)
 void arch_cpu_idle_dead(void)
-	sched_preempt_enable_no_resched();
 	 * Disable on the down path. This will be re-enabled by
 	 * start_secondary() via start_secondary_resume() below

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