Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception

[Masami Hiramatsu]

Hi Mark,

On Fri, 19 Jul 2019 10:59:59 +0100
Mark Rutland <mark.rutland@xxxxxxx> wrote:

> 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.rutland@xxxxxxx> 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).

On x86, it uses rcu_nmi_enter/exit() for kernel mode. For user mode,
we don't need to care since it must not be an idle task.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>