Re: [PATCH tip/core/urgent] rcu: rcu_read_lock_bh_held(): disablingirqs also disables bh

[Lai Jiangshan]

On 09/23/2010 08:32 AM, Paul E. McKenney wrote:
> rcu_dereference_bh() doesnt know yet about hard irq being disabled, so
> lockdep can trigger in netpoll_rx() after commit f0f9deae9e7c4 (netpoll:
> Disable IRQ around RCU dereference in netpoll_rx)
> 
> Reported-by: Miles Lane <miles.lane@xxxxxxxxx>
> Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> Tested-by: Miles Lane <miles.lane@xxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/rcupdate.h |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 9fbc54a..0dcc00e 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -454,7 +454,8 @@ static inline notrace void rcu_read_unlock_sched_notrace(void)
>   * Makes rcu_dereference_check() do the dirty work.
>   */
>  #define rcu_dereference_bh(p) \
> -		rcu_dereference_check(p, rcu_read_lock_bh_held())
> +		rcu_dereference_check(p, rcu_read_lock_bh_held() || \
> +					 irqs_disabled())
>  
>  /**
>   * rcu_dereference_sched - fetch RCU-protected pointer, checking for RCU-sched

In -rt, rcu_read_lock_bh() is preemptible, but all RCU Implementation,
PREEMPT_RCU, TREE_PREEMPT_RCU, TINY_PREEMPT_RCU, do not guarantee
hard-irq/irq-disabled contexts are preeemptible-rcu-read-site critical regions,
so it(disabling irqs also disables bh) is not true in -rt I think,
To make the codes be still safe in the future(-rt), I strongly recommend to
use rcu_read_lock_bh() as needed even irq disabled.

Current, preempt_disable()/rcu_read_lock_sched() do not guarantee that
they implies rcu_read_lock() either.

But in future, the rcu may give these guarantees, it's a plan of mine.

Lai
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