Re: RCU lockup? (was: Re: [PATCH v2 tip/core/rcu 10/14] rcu: Don't redundantly disable irqs in rcu_irq_{enter,exit}())

[Geert Uytterhoeven]

Hi Paul,

On Thu, Jan 21, 2016 at 5:06 PM, Paul E. McKenney
<paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Jan 21, 2016 at 02:22:56PM +0100, Geert Uytterhoeven wrote:
>> On Thu, Dec 10, 2015 at 12:10 AM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > This commit replaces a local_irq_save()/local_irq_restore() pair with
>> > a lockdep assertion that interrupts are already disabled.  This should
>> > remove the corresponding overhead from the interrupt entry/exit fastpaths.
>> >
>> > This change was inspired by the fact that Iftekhar Ahmed's mutation
>> > testing showed that removing rcu_irq_enter()'s call to local_ird_restore()
>> > had no effect, which might indicate that interrupts were always enabled
>> > anyway.
>> >
>> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
>> > ---
>> >  include/linux/rcupdate.h   |  4 ++--
>> >  include/linux/rcutiny.h    |  8 ++++++++
>> >  include/linux/rcutree.h    |  2 ++
>> >  include/linux/tracepoint.h |  4 ++--
>> >  kernel/rcu/tree.c          | 32 ++++++++++++++++++++++++++------
>> >  5 files changed, 40 insertions(+), 10 deletions(-)
>>
>> This commit (7c9906ca5e582a773fff696975e312cef58a7386) is triggering lock ups
>> during boot on r8a7791/koelsch (dual Cortex A15). Probably this commit does not
>> contain the real bug, but a symptom.
>
> On the off-chance that it is related, here is Ding Tianhong's patch
> that addressed some lockups:
>
> http://www.eenyhelp.com/patch-rfc-locking-mutexes-dont-spin-owner-when-wait-list-not-null-help-215929641.html
>
> Does that help in your case?

Unfortunately not.

>> Unfortunately I cannot reproduce it with CONFIG_PROVE_RCU=y.
>>
>> I started seeing the issue when disabling an innocent option in
>> shmobile_defconfig. I tracked it down to the removal of an unused C function,
>> containing hardware support for another system. Replacing the C function by
>> a dummy function with the right number of "asm("nop")"s (depending on kernel
>> version and/or kernel config, sigh) made the issue go away.
>> Adding or removing nops makes the issue reappear, and has some impact on
>> how early the issue happens (sometimes as late as early userspace).
>> Adding a multiple of 16 nops has no impact.
>> So it looks like something that should be cacheline-aligned isn't...
>
> The other possibility is that it is timing related.  Either way, fun
> to find...
>
>> CONFIG_TREE_RCU=y
>>
>> Do you have a suggestion?
>
> Only trying Ding's patch...

Thanks for the pointer anyway!

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds