On Thu, Jul 18, 2019 at 11:31:33PM +0900, Masami Hiramatsu wrote:
> On Thu, 18 Jul 2019 10:20:23 +0100
> Mark Rutland <mark.rutl...@arm.com> wrote:
>
> > On Wed, Jul 17, 2019 at 11:22:15PM -0700, Paul E. McKenney wrote:
> > > On Thu, Jul 18, 2019 at 02:43:58PM +0900, Masami Hiramatsu wrote:
> > > > Remove rcu_read_lock()/rcu_read_unlock() from debug exception
> > > > handlers since the software breakpoint can be hit on idle task.
> >
> > Why precisely do we need to elide these? Are we seeing warnings today?
>
> Yes, unfortunately, or fortunately. Naresh reported that warns when
> ftracetest ran. I confirmed that happens if I probe on default_idle_call too.
>
> /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
> /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
> /sys/kernel/debug/tracing # [ 135.122237]
> [ 135.125035] =============================
> [ 135.125310] WARNING: suspicious RCU usage
> [ 135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
> [ 135.125904] -----------------------------
> [ 135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally
> while idle!
> [ 135.126839]
> [ 135.126839] other info that might help us debug this:
> [ 135.126839]
> [ 135.127410]
> [ 135.127410] RCU used illegally from idle CPU!
> [ 135.127410] rcu_scheduler_active = 2, debug_locks = 1
> [ 135.128114] RCU used illegally from extended quiescent state!
> [ 135.128555] 1 lock held by swapper/0/0:
> [ 135.128944] #0: (____ptrval____) (rcu_read_lock){....}, at:
> call_break_hook+0x0/0x178
> [ 135.130499]
> [ 135.130499] stack backtrace:
> [ 135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted
> 5.2.0-08445-g9187c508bdc7 #20
> [ 135.131841] Hardware name: linux,dummy-virt (DT)
> [ 135.132224] Call trace:
> [ 135.132491] dump_backtrace+0x0/0x140
> [ 135.132806] show_stack+0x24/0x30
> [ 135.133133] dump_stack+0xc4/0x10c
> [ 135.133726] lockdep_rcu_suspicious+0xf8/0x108
> [ 135.134171] call_break_hook+0x170/0x178
> [ 135.134486] brk_handler+0x28/0x68
> [ 135.134792] do_debug_exception+0x90/0x150
> [ 135.135051] el1_dbg+0x18/0x8c
> [ 135.135260] default_idle_call+0x0/0x44
> [ 135.135516] cpu_startup_entry+0x2c/0x30
> [ 135.135815] rest_init+0x1b0/0x280
> [ 135.136044] arch_call_rest_init+0x14/0x1c
> [ 135.136305] start_kernel+0x4d4/0x500
> [ 135.136597]
>
> > > The exception entry and exit use irq_enter() and irq_exit(), in this
> > > case, correct? Otherwise RCU will be ignoring this CPU.
> >
> > This is missing today, which sounds like the underlying bug.
>
> Agreed. I'm not so familier with how debug exception is handled on arm64,
> would it be a kind of NMI or IRQ?
They're more like faults, in that they're synchronous exceptions.
Given that, I think using irq_enter() / irq_exit() would be surprising
here, but perhaps they're misnamed.
What do other architectures do here? Having a kprobe on the critical
path to idle doesn't sound specific to arm64, but perhaps it is (and we
should rule it out).
Thanks,
Mark.
