From: Mark Rutland <mark.rutland@arm.com>
To: Masami Hiramatsu <mhiramat@kernel.org>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Will Deacon <will.deacon@arm.com>,
linux-arm-kernel@lists.infradead.org,
linux-kernel@vger.kernel.org,
Naresh Kamboju <naresh.kamboju@linaro.org>,
Dan Rue <dan.rue@linaro.org>, Matt Hart <matthew.hart@linaro.org>,
Anders Roxell <anders.roxell@linaro.org>,
Daniel Diaz <daniel.diaz@linaro.org>,
James Morse <james.morse@arm.com>
Subject: Re: [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception
Date: Wed, 24 Jul 2019 11:45:19 +0100 [thread overview]
Message-ID: <20190724104518.GA2624@lakrids.cambridge.arm.com> (raw)
In-Reply-To: <20190720165458.7333b65244312843c2ca6857@kernel.org>
On Sat, Jul 20, 2019 at 04:54:58PM +0900, Masami Hiramatsu wrote:
> Hi Mark,
>
> On Fri, 19 Jul 2019 10:59:59 +0100
> Mark Rutland <mark.rutland@arm.com> wrote:
>
> > On Thu, Jul 18, 2019 at 11:31:33PM +0900, Masami Hiramatsu wrote:
> > > On Thu, 18 Jul 2019 10:20:23 +0100
> > > Mark Rutland <mark.rutland@arm.com> wrote:
> > >
> > > > 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?
> > >
> > > Yes, unfortunately, or fortunately. Naresh reported that warns when
> > > ftracetest ran. I confirmed that happens if I probe on default_idle_call too.
> > >
> > > /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]
> > >
> > > > > 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.
> > >
> > > Agreed. I'm not so familier with how debug exception is handled on arm64,
> > > would it be a kind of NMI or IRQ?
> >
> > They're more like faults, in that they're synchronous exceptions.
> >
> > Given that, I think using irq_enter() / irq_exit() would be surprising
> > here, but perhaps they're misnamed.
> >
> > What do other architectures do here? Having a kprobe on the critical
> > path to idle doesn't sound specific to arm64, but perhaps it is (and we
> > should rule it out).
>
> On x86, it uses rcu_nmi_enter/exit() for kernel mode. For user mode,
> we don't need to care since it must not be an idle task.
Ok. IIUC, doing the same for arm64 would make sense.
Thanks,
Mark.
prev parent reply other threads:[~2019-07-24 10:45 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-18 5:43 [PATCH 0/3] arm64: kprobes: Fix some bugs in arm64 kprobes Masami Hiramatsu
2019-07-18 5:43 ` [PATCH 1/3] arm64: kprobes: Recover pstate.D in single-step exception handler Masami Hiramatsu
2019-07-19 10:07 ` James Morse
2019-07-20 6:22 ` Masami Hiramatsu
2019-07-18 5:43 ` [PATCH 2/3] arm64: unwind: Prohibit probing on return_address() Masami Hiramatsu
2019-07-18 5:43 ` [PATCH 3/3] arm64: debug: Remove rcu_read_lock from debug exception Masami Hiramatsu
2019-07-18 6:22 ` Paul E. McKenney
2019-07-18 9:20 ` Mark Rutland
2019-07-18 14:31 ` Masami Hiramatsu
2019-07-19 8:42 ` James Morse
2019-07-20 7:32 ` Masami Hiramatsu
2019-07-21 1:50 ` Masami Hiramatsu
2019-07-19 9:59 ` Mark Rutland
2019-07-20 7:54 ` Masami Hiramatsu
2019-07-24 10:45 ` Mark Rutland [this message]
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