From: Nathan Lynch nathanl@linux.ibm.com
[ Upstream commit 787252a10d9422f3058df9a4821f389e5326c440 ]
With PREEMPT_COUNT=y, when a CPU is offlined and then onlined again, we get:
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: dump_stack_lvl+0xac/0x108 __schedule_bug+0xac/0xe0 __schedule+0xcf8/0x10d0 schedule_idle+0x3c/0x70 do_idle+0x2d8/0x4a0 cpu_startup_entry+0x38/0x40 start_secondary+0x2ec/0x3a0 start_secondary_prolog+0x10/0x14
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 nathanl@linux.ibm.com Reviewed-by: Valentin Schneider valentin.schneider@arm.com Reviewed-by: Srikar Dronamraju srikar@linux.vnet.ibm.com Signed-off-by: Michael Ellerman mpe@ellerman.id.au Link: https://lore.kernel.org/r/20211015173902.2278118-1-nathanl@linux.ibm.com Signed-off-by: Sasha Levin sashal@kernel.org --- 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 91f274134884..452cbf98bfd7 100644 --- a/arch/powerpc/kernel/smp.c +++ b/arch/powerpc/kernel/smp.c @@ -1578,8 +1578,6 @@ 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