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

Paul E. McKenney
2019-07-22T12:07:44Z

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 <naresh.kamboju@linaro.org>
> Cc: Paul E. McKenney <paulmck@linux.ibm.com>

From an RCU viewpoint:

Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>

> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
> 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();
> }