On Sat, 20 Jul 2019 16:32:32 +0900 Masami Hiramatsu <mhira...@kernel.org> wrote:
> Hi James, > > On Fri, 19 Jul 2019 09:42:05 +0100 > James Morse <james.mo...@arm.com> wrote: > > > Hi, > > > > On 7/18/19 3:31 PM, Masami Hiramatsu wrote: > > > On Thu, 18 Jul 2019 10:20:23 +0100 > > > Mark Rutland <mark.rutl...@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 > > > > > [ 135.132224] 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 > > > > >>> 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? > > > > Debug exceptions can interrupt both SError (think: machine check) and > > pseudo-NMI, which both in turn interrupt interrupt-masked code. So they > > are a kind of NMI. But, be careful not to call 'nmi_enter()' twice, see > > do_serror() for how we work around this... > > OK. I think we can use rcu_nmi_enter/exit() as same as x86. Adding this solves rcu_read_lock() warning issues too. So I will just replace [PATCH 3/3] with that. > > > Anyway, it seems that normal irqs are also not calling irq_enter/exit > > > except for arch/arm64/kernel/smp.c > > drivers/irqchip/irq-gic.c:gic_handle_irq() either calls > > handle_domain_irq() or handle_IPI(). The enter/exit calls live in those > > functions. > > Ah, I see. > Would you think we need to put rcu_nmi_enter/exit() as similar to x86 > on do_mem_abort() and do_sp_pc_abort() too? Hmm, it seems that adding rcu_nmi_enter/exit to both function causes a failure of init process. At this moment I don't do that. Thank you, -- Masami Hiramatsu <mhira...@kernel.org>
