Messages in this thread | | | Date | Mon, 22 Jul 2019 05:07:35 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU |
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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(); > } >
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