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?
Anyway, it seems that normal irqs are also not calling irq_enter/exit
except for arch/arm64/kernel/smp.c. We need to fix that too for avoiding
unexpected RCU issues.
Thank you,
--
Masami Hiramatsu <mhira...@kernel.org>
