Subject: Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU

From: "Paul E. McKenney" <[email-masked]>
Date: Mon, 22 Jul 2019 05:07:35 -0700

On Mon, Jul 22, 2019 at 04:48:58PM +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 <[email-masked]>
> Cc: Paul E. McKenney <[email-masked]>

>From an RCU viewpoint:

Acked-by: Paul E. McKenney <[email-masked]>

> Signed-off-by: Masami Hiramatsu <[email-masked]>
> ---
>  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..a6b244240db6 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.
> + * 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 in_nmi() to return true,
> +		 * but we need to notify RCU.
> +		 */
> +		rcu_nmi_enter();
> +	}
> +
> +	preempt_disable();
> +
> +	/* This code is a bit fragile.  Test it. */
> +	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
> +}
> +NOKPROBE_SYMBOL(debug_exception_enter);
> +
> +static void debug_exception_exit(struct pt_regs *regs)
> +{
> +	preempt_enable_no_resched();
> +
> +	if (!user_mode(regs))
> +		rcu_nmi_exit();
> +}
> +NOKPROBE_SYMBOL(debug_exception_exit);
> +
>  #ifdef CONFIG_ARM64_ERRATUM_1463225
>  DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
>  
> @@ -824,6 +860,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
>  	if (interrupts_enabled(regs))
>  		trace_hardirqs_off();
>  
> +	debug_exception_enter(regs);
> +
>  	if (user_mode(regs) && !is_ttbr0_addr(pc))
>  		arm64_apply_bp_hardening();
>  
> @@ -832,6 +870,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
>  				 inf->sig, inf->code, (void __user *)pc, esr);
>  	}
>  
> +	debug_exception_exit(regs);
> +
>  	if (interrupts_enabled(regs))
>  		trace_hardirqs_on();
>  }
> 

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