Re: [PATCH v3 3/4] arm64: Make debug exception handlers visible from RCU

[Masami Hiramatsu <mhiramat@kernel.org>]

Hi Will,

On Wed, 31 Jul 2019 18:26:03 +0100
Will Deacon <will@kernel.org> wrote:

> On Thu, Jul 25, 2019 at 05:16:15PM +0900, Masami Hiramatsu wrote:
> > Make debug exceptions visible from RCU so that synchronize_rcu()
> > correctly track the debug exception handler.
> > 
> > This also introduces sanity checks for user-mode exceptions as same
> > as x86's ist_enter()/ist_exit().
> > 
> > The debug exception can interrupt in idle task. For example, it warns
> > if we put a kprobe on a function called from idle task as below.
> > The warning message showed that the rcu_read_lock() caused this
> > problem. But actually, this means the RCU is lost the context which
> > is already in NMI/IRQ.
> > 
> >   /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]
> > 
> > So make debug exception visible to RCU can fix this warning.
> > 
> > Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> > Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > ---
> >  Changes in v3:
> >   - Make a comment for debug_exception_enter() clearer.
> > ---
> >  arch/arm64/mm/fault.c |   40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> > 
> > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > index 9568c116ac7f..ed6c55c87fdc 100644
> > --- a/arch/arm64/mm/fault.c
> > +++ b/arch/arm64/mm/fault.c
> > @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
> >  	debug_fault_info[nr].name	= name;
> >  }
> >  
> > +/*
> > + * In debug exception context, we explicitly disable preemption.
> 
> Maybe add "despite having interrupts disabled"?

OK, I'll add it.

> > + * This serves two purposes: it makes it much less likely that we would
> > + * accidentally schedule in exception context and it will force a warning
> > + * if we somehow manage to schedule by accident.
> > + */
> > +static void debug_exception_enter(struct pt_regs *regs)
> > +{
> > +	if (user_mode(regs)) {
> > +		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> > +	} else {
> > +		/*
> > +		 * We might have interrupted pretty much anything.  In
> > +		 * fact, if we're a debug exception, we can even interrupt
> > +		 * NMI processing. We don't want this code makes in_nmi()
> > +		 * to return true, but we need to notify RCU.
> > +		 */
> > +		rcu_nmi_enter();
> > +	}
> > +
> > +	preempt_disable();
> 
> If you're addingt new functions for entry/exit, maybe move the
> trace_hardirqs_{on,off}() calls in here too?

OK, let's move it in these functions.

Thank you!

> 
> Will


-- 
Masami Hiramatsu <mhiramat@kernel.org>