Re: [PATCH -v5 6/5] context_tracking: fix exception_enter when already in IN_KERNEL

Paul E. McKenney
2015-02-11T21:27:31Z

On Wed, Feb 11, 2015 at 02:43:19PM -0500, Rik van Riel wrote:
> If exception_enter happens when already in IN_KERNEL state, the
> code still calls context_tracking_exit, which ends up in
> rcu_eqs_exit_common, which explodes with a WARN_ON when it is
> called in a situation where dynticks are not enabled.
>
> This can be avoided by having exception_enter only switch to
> IN_KERNEL state if the current state is not already IN_KERNEL.

Ugh... Time to formally verify, sounds like...

Thanx, Paul

> Signed-off-by: Rik van Riel <riel@redhat.com>
> Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
> ---
> Frederic, you will want this "bonus" patch, too :)
>
> Thanks to Luiz for finding this one. Whatever I was running did not
> trigger this issue...
>
> include/linux/context_tracking.h | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index b65fd1420e53..9da230406e8c 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -37,7 +37,8 @@ static inline enum ctx_state exception_enter(void)
> return 0;
>
> prev_ctx = this_cpu_read(context_tracking.state);
> - context_tracking_exit(prev_ctx);
> + if (prev_ctx != IN_KERNEL)
> + context_tracking_exit(prev_ctx);
>
> return prev_ctx;
> }
>