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

[Masami Hiramatsu]

On Sat, 20 Jul 2019 16:32:32 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> Hi James,
> 
> On Fri, 19 Jul 2019 09:42:05 +0100
> James Morse <james.morse@xxxxxxx> wrote:
> 
> > Hi,
> > 
> > On 7/18/19 3:31 PM, 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
> > 
> > > [  135.132224] 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
> > 
> > >>> 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?
> > 
> > Debug exceptions can interrupt both SError (think: machine check) and 
> > pseudo-NMI, which both in turn interrupt interrupt-masked code. So they 
> > are a kind of NMI. But, be careful not to call 'nmi_enter()' twice, see 
> > do_serror() for how we work around this...
> 
> OK. I think we can use rcu_nmi_enter/exit() as same as x86.

Adding this solves rcu_read_lock() warning issues too.
So I will just replace [PATCH 3/3] with that.

> > > Anyway, it seems that normal irqs are also not calling irq_enter/exit
> > > except for arch/arm64/kernel/smp.c
> > drivers/irqchip/irq-gic.c:gic_handle_irq() either calls 
> > handle_domain_irq() or handle_IPI(). The enter/exit calls live in those 
> > functions.
> 
> Ah, I see.
> Would you think we need to put rcu_nmi_enter/exit() as similar to x86
> on do_mem_abort() and do_sp_pc_abort() too?

Hmm, it seems that adding rcu_nmi_enter/exit to both function causes
a failure of init process. At this moment I don't do that.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>