Re: [PATCH tip/core/rcu 01/48] rcu: optionally leave lockdep enabled after RCU lockdep splat

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> From: Lai Jiangshan <laijs@cn.fujitsu.com>
> 
> There is no need to disable lockdep after an RCU lockdep splat,
> so remove the debug_lockdeps_off() from lockdep_rcu_dereference().
> To avoid repeated lockdep splats, use a static variable in the inlined
> rcu_dereference_check() and rcu_dereference_protected() macros so that
> a given instance splats only once, but so that multiple instances can
> be detected per boot.
> 
> This is controlled by a new config variable CONFIG_PROVE_RCU_REPEATEDLY,
> which is disabled by default.  This provides the normal lockdep behavior
> by default, but permits people who want to find multiple RCU-lockdep
> splats per boot to easily do so.

I'll play the devil's advocate here. (just because that's so much fun)
;-)

If we look at:

include/linux/debug_locks.h:

static inline int __debug_locks_off(void)
{
        return xchg(&debug_locks, 0);
}

We see that all code following a call to "debug_locks_off()" can assume
that it cannot possibly run concurrently with other code following
"debug_locks_off()". Now, I'm not saying that the code we currently have
will necessarily break, but I think it is worth asking if there is any
assumption in lockdep (or RCU lockdep more specifically) about mutual
exclusion after debug_locks_off() ?

Because if there is, then this patch is definitely breaking something by
not protecting lockdep against multiple concurrent executions.

Thanks,

Mathieu

> 
> Requested-by: Eric Paris <eparis@redhat.com>
> Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> Tested-by: Eric Paris <eparis@redhat.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
>  include/linux/rcupdate.h |    6 ++----
>  kernel/lockdep.c         |    2 ++
>  lib/Kconfig.debug        |   12 ++++++++++++
>  3 files changed, 16 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index a8b2e03..4dca275 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -230,8 +230,7 @@ extern int rcu_my_thread_group_empty(void);
>   */
>  #define rcu_dereference_check(p, c) \
>  	({ \
> -		if (debug_lockdep_rcu_enabled() && !(c)) \
> -			lockdep_rcu_dereference(__FILE__, __LINE__); \
> +		__do_rcu_dereference_check(c); \
>  		rcu_dereference_raw(p); \
>  	})
>  
> @@ -248,8 +247,7 @@ extern int rcu_my_thread_group_empty(void);
>   */
>  #define rcu_dereference_protected(p, c) \
>  	({ \
> -		if (debug_lockdep_rcu_enabled() && !(c)) \
> -			lockdep_rcu_dereference(__FILE__, __LINE__); \
> +		__do_rcu_dereference_check(c); \
>  		(p); \
>  	})
>  
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 2594e1c..73747b7 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -3801,8 +3801,10 @@ void lockdep_rcu_dereference(const char *file, const int line)
>  {
>  	struct task_struct *curr = current;
>  
> +#ifndef CONFIG_PROVE_RCU_REPEATEDLY
>  	if (!debug_locks_off())
>  		return;
> +#endif /* #ifdef CONFIG_PROVE_RCU_REPEATEDLY */
>  	printk("\n===================================================\n");
>  	printk(  "[ INFO: suspicious rcu_dereference_check() usage. ]\n");
>  	printk(  "---------------------------------------------------\n");
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 935248b..94090b4 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -512,6 +512,18 @@ config PROVE_RCU
>  
>  	 Say N if you are unsure.
>  
> +config PROVE_RCU_REPEATEDLY
> +	bool "RCU debugging: don't disable PROVE_RCU on first splat"
> +	depends on PROVE_RCU
> +	default n
> +	help
> +	 By itself, PROVE_RCU will disable checking upon issuing the
> +	 first warning (or "splat").  This feature prevents such
> +	 disabling, allowing multiple RCU-lockdep warnings to be printed
> +	 on a single reboot.
> +
> +	 Say N if you are unsure.
> +
>  config LOCKDEP
>  	bool
>  	depends on DEBUG_KERNEL && TRACE_IRQFLAGS_SUPPORT && STACKTRACE_SUPPORT && LOCKDEP_SUPPORT
> -- 
> 1.7.0
> 

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com