Re: [PATCH] s390/smp: Move rcu_cpu_starting() earlier

From: Qian Cai
Date: Sat Oct 31 2020 - 19:39:02 EST


On Sat, 2020-10-31 at 19:37 +0100, Heiko Carstens wrote:
> On Wed, Oct 28, 2020 at 02:27:42PM -0400, Qian Cai wrote:
> > The call to rcu_cpu_starting() in smp_init_secondary() is not early
> > enough in the CPU-hotplug onlining process, which results in lockdep
> > splats as follows:
> >
> > WARNING: suspicious RCU usage
> > -----------------------------
> > kernel/locking/lockdep.c:3497 RCU-list traversed in non-reader section!!
> >
> > other info that might help us debug this:
> >
> > RCU used illegally from offline CPU!
> > rcu_scheduler_active = 1, debug_locks = 1
> > no locks held by swapper/1/0.
> >
> > Call Trace:
> > show_stack+0x158/0x1f0
> > dump_stack+0x1f2/0x238
> > __lock_acquire+0x2640/0x4dd0
> > lock_acquire+0x3a8/0xd08
> > _raw_spin_lock_irqsave+0xc0/0xf0
> > clockevents_register_device+0xa8/0x528
> > init_cpu_timer+0x33e/0x468
> > smp_init_secondary+0x11a/0x328
> > smp_start_secondary+0x82/0x88
> >
> > This is avoided by moving the call to rcu_cpu_starting up near the
> > beginning of the smp_init_secondary() function. Note that the
> > raw_smp_processor_id() is required in order to avoid calling into
> > lockdep before RCU has declared the CPU to be watched for readers.
> >
> > Link: https://lore.kernel.org/lkml/160223032121.7002.1269740091547117869.tip-bot2@tip-bot2/
> > Signed-off-by: Qian Cai <cai@xxxxxxxxxx>
> > ---
> > arch/s390/kernel/smp.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
>
> Could you provide the config you used? I'm wondering why I can't
> reproduce this even though I have lot's of debug options enabled.
https://cailca.coding.net/public/linux/mm/git/files/master/s390.config

Essentially, I believe it requires CONFIG_PROVE_RCU_LIST=y. Also, it occurs to
me that this only starts to happen after the commit mentioned in the above link.