Re: [PATCH] sched: Teach might_sleep about preemptable rcu

Paul E. McKenney
2009-12-14T23:03:47Z

On Mon, Dec 14, 2009 at 11:44:32PM +0100, Frederic Weisbecker wrote:
> In practice, it is harmless to voluntarily sleep in a rcu_read_lock()
> section if we are running under preempt rcu, but it is illegal because
> if we build a kernel running non-preemptable rcu.
>
> Currently, might_sleep() doesn't notice sleepable operations under
> rcu_read_lock() sections if we are running under preemptable rcu
> because preempt_count() is left untouched after rcu_read_lock() in
> this case. But we want developers who test their changes under such
> config to notice the "sleeping while atomic" issues.
>
> Then we add rcu_read_lock_nesting to prempt_count() in might_sleep()
> checks.

Cute!!!

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> ---
> include/linux/rcutree.h | 11 +++++++++++
> kernel/sched.c | 2 +-
> 2 files changed, 12 insertions(+), 1 deletions(-)
>
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index c93eee5..8044b1b 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -45,6 +45,12 @@ extern void __rcu_read_unlock(void);
> extern void synchronize_rcu(void);
> extern void exit_rcu(void);
>
> +/*
> + * Defined as macro as it is a very low level header
> + * included from areas that don't even know about current
> + */
> +#define rcu_preempt_depth() (current->rcu_read_lock_nesting)
> +
> #else /* #ifdef CONFIG_TREE_PREEMPT_RCU */
>
> static inline void __rcu_read_lock(void)
> @@ -63,6 +69,11 @@ static inline void exit_rcu(void)
> {
> }
>
> +static inline int rcu_preempt_depth(void)
> +{
> + return 0;
> +}
> +
> #endif /* #else #ifdef CONFIG_TREE_PREEMPT_RCU */
>
> static inline void __rcu_read_lock_bh(void)
> diff --git a/kernel/sched.c b/kernel/sched.c
> index ab42754..586c82c 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -9658,7 +9658,7 @@ void __init sched_init(void)
> #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
> static inline int preempt_count_equals(int preempt_offset)
> {
> - int nested = preempt_count() & ~PREEMPT_ACTIVE;
> + int nested = (preempt_count() & ~PREEMPT_ACTIVE) + rcu_preempt_depth();
>
> return (nested == PREEMPT_INATOMIC_BASE + preempt_offset);
> }
> --
> 1.6.2.3
>