Messages in this thread | | | Date | Wed, 24 Jul 2019 20:47:48 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH v2 3/4] arm64: Make debug exception handlers visible from RCU |
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On Tue, 23 Jul 2019 18:07:56 +0100 James Morse <james.morse@arm.com> wrote:
> Hi, > > On 22/07/2019 08:48, 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. > > > So make debug exception visible to RCU can fix this warning. > > > 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"); > > Would moving entry.S's context_tracking_user_exit() call to be before do_debug_exception() > also fix this?
It sounds like treating only user context, correct? This part is just adding assertion, not fixing the problem which Naresh reported.
> > I don't know the reason its done 'after' debug exception handling. Its always been like > this: commit 6c81fe7925cc4c42 ("arm64: enable context tracking"). > > > > + } 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. > > How come? If you interrupted an SError or pseudo-nmi, it already is. Those paths should > all be painted no-kprobe, but I'm sure there are gaps. The hw-breakpoints can almost > certainly hook them.
I think that sentense means "we don't want that this code makes in_nmi() to return true" So, if the breakpoint interrupts pNMI/SError context, it is OK that in_nmi() returns true.
> > > > + */ > > + rcu_nmi_enter(); > > Can we interrupt printk()? Do we need printk_nmi_enter()? ... What about ftrace?
Good point! As far as I know, we don't use it because ftrace doesn't use printk. But indeed, kprobes user can use printk and they have to call printk_nmi_enter()/exit(), that must be commented in the documentation. Anyway, basically it is user's choice.
> > Because SError and pseudo-nmi can interrupt interrupt-masked code, we describe them as > NMI. The only difference here is these exceptions are synchronous. > > > I suspect we should make these debug exceptions nmi for EL1. We can then use this for the > kprobe-re-entrance stuff so the pre/post hooks don't get run if they interrupted something > also described as NMI.
I'm not sure how it can prevent... anyway because we have to run a single-stepping for recovery, and kprobe already check the reentered kprobes and skip user-handlers in such case.
Thank you,
> > > > + } > > + > > + 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); > > > Thanks, > > James
-- Masami Hiramatsu <mhiramat@kernel.org>
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