Re: [PATCH] Teach RCU that idle task is not quiscent state at boot

[Ingo Molnar <mingo@elte.hu>]

* Paul E. McKenney <paulmck@linux.vnet.ibm.com> wrote:

> This patch fixes a bug located by Vegard Nossum with the aid of
> kmemcheck.
> 
> The boot CPU runs in the context of its idle thread during boot-up.
> During this time, idle_cpu(0) will always return nonzero, which will
> fool Classic and Hierarchical RCU into deciding that a large chunk of
> the boot-up sequence is a big long quiescent state.  This in turn causes
> RCU to prematurely end grace periods during this time.
> 
> This patch changes the rcutree.c and rcuclassic.c rcu_check_callbacks()
> function to ignore the idle tasks as a quiescent state until the
> system_state is no longer SYSTEM_BOOTING.  After this point, the idle
> task really does represent a quiescent state.
> 
> In addition, this patch takes Nick Piggin's suggestion to make the
> system_state global variable be __read_mostly.
> 
> Located-by: Vegard Nossum <vegard.nossum@gmail.com>
> Tested-by: Vegard Nossum <vegard.nossum@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> 
>  init/main.c         |    2 +-
>  kernel/rcuclassic.c |    4 ++--
>  kernel/rcutree.c    |    4 ++--
>  3 files changed, 5 insertions(+), 5 deletions(-)

Applied to tip:core/urgent, thanks Paul!

	Ingo