In message: [linux-yocto][v5.4/standard/preempt-rt][PATCH] sched: Remove
unexpected mmdrop on RT
on 15/07/2020 Yongxin Liu wrote:
> mmdrop() in finish_cpu() was introduced on RT due to the merge of
> commit f7757368e0f0 ("sched/core: Fix illegal RCU from offline CPUs").
>
> However, it will cause kernel panic on RT kernel, sinces it breaks
> commit a9647f8b0741 ("sched: Move mmdrop to RCU on RT"), which uses
> delayed mmdrop on RT.
>
> Without this patch, the following fatal exception will show after running
> "rtcwake -m mem -s 10" several times.
Aha. That's why it didn't show up in my boot tests. I didn't hit
the right path.
I've merged the changes, and the SRCREV updates will be in my next
pull request.
Bruce
>
> general protection fault: 0000 [#1] PREEMPT_RT SMP PTI
> RIP: 0010:switch_mm_irqs_off+0x14c/0x930
>
> Call Trace:
> __schedule+0x2f6/0x960
> ? __schedule+0x2f6/0x960
> ? preempt_schedule+0x38/0x40
> preempt_schedule_common+0x27/0xf0
> preempt_schedule+0x38/0x40
> ___preempt_schedule+0x16/0x20
> ? migrate_enable+0x116/0x330
> ? migrate_enable+0x116/0x330
> migrate_enable+0x12a/0x330
> rt_spin_unlock+0x2f/0xa0
> serial8250_console_write+0x14b/0x310
> ? vt_console_print+0x217/0x3e0
> univ8250_console_write+0x21/0x30
> printk_kthread_func+0x272/0x610
> kthread+0x12f/0x150
> ? console_cpu_notify+0x30/0x30
> ? kthread_park+0x90/0x90
> ret_from_fork+0x3a/0x50
> Modules linked in:
>
> Kernel panic - not syncing: Fatal exception
>
> Signed-off-by: Yongxin Liu <[email protected]>
> ---
> kernel/cpu.c | 1 -
> 1 file changed, 1 deletion(-)
>
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 2b6aec3623c1..c7fa9498e348 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -576,7 +576,6 @@ static int finish_cpu(unsigned int cpu)
> */
> if (mm != &init_mm)
> idle->active_mm = &init_mm;
> - mmdrop(mm);
> return 0;
> }
>
> --
> 2.14.4
>
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