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

From: Mark Rutland <[email-masked]>
Date: Thu, 18 Jul 2019 10:20:23 +0100

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?

> 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.

Thanks,
Mark.

> 
> 							Thanx, Paul
> 
> > Actually, we don't need it because those handlers run in exception
> > context where the interrupts are disabled. This means those are never
> > preempted.
> > 
> > Reported-by: Naresh Kamboju <[email-masked]>
> > Cc: Paul E. McKenney <[email-masked]>
> > Signed-off-by: Masami Hiramatsu <[email-masked]>
> > ---
> >  arch/arm64/kernel/debug-monitors.c |   14 ++++++++------
> >  1 file changed, 8 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> > index f8719bd30850..48222a4760c2 100644
> > --- a/arch/arm64/kernel/debug-monitors.c
> > +++ b/arch/arm64/kernel/debug-monitors.c
> > @@ -207,16 +207,16 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
> >  
> >  	list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
> >  
> > -	rcu_read_lock();
> > -
> > +	/*
> > +	 * Since single-step exception disables interrupt, this function is
> > +	 * entirely not preemptible, and we can use rcu list safely here.
> > +	 */
> >  	list_for_each_entry_rcu(hook, list, node)	{
> >  		retval = hook->fn(regs, esr);
> >  		if (retval == DBG_HOOK_HANDLED)
> >  			break;
> >  	}
> >  
> > -	rcu_read_unlock();
> > -
> >  	return retval;
> >  }
> >  NOKPROBE_SYMBOL(call_step_hook);
> > @@ -305,14 +305,16 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
> >  
> >  	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
> >  
> > -	rcu_read_lock();
> > +	/*
> > +	 * Since brk exception disables interrupt, this function is
> > +	 * entirely not preemptible, and we can use rcu list safely here.
> > +	 */
> >  	list_for_each_entry_rcu(hook, list, node) {
> >  		unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
> >  
> >  		if ((comment & ~hook->mask) == hook->imm)
> >  			fn = hook->fn;
> >  	}
> > -	rcu_read_unlock();
> >  
> >  	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
> >  }
> > 

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