Re: [PATCH] tree/tiny rcu: Add debug RCU head option (v2)

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Thu, Mar 18, 2010 at 09:30:24PM -0400, Mathieu Desnoyers wrote:
> Poisoning the rcu_head callback list.
> 
> Helps finding racy users of call_rcu(), which results in hangs because list
> entries are overwritten and/or skipped.
> 
> A lot of non-initialized rcu list heads will be found with this option enabled.
> It is important to do not just blindly initialize them before each call_rcu, but
> rather to perform the initialization at the proper location, after the memory
> has been allocated, so we can effectively detect incorrect call_rcu users.
> 
> This patch version does not use "debug objects", although it probably should.
> Some day I might find time to look into this transition, but the patch is usable
> as is.
> 
> The weird #ifdef in the networking code comes from an agreement with Eric
> Dumazet about how to disable the build check for the networking code.  For those
> who wonder about the existence of this build-time check: it generates a build
> error when the size of struct dst_entry grows (because it is very performance
> sensitive). So the agreement here has been to disable the check when the
> DEBUG_RCU_HEAD config option is active.

If this one finds at least one bug (or if the previous version found
at least one bug -- bad memory day here), then I will push it.  It can
always be improved later, right?

The patch looks good to me.

							Thanx, Paul

> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> CC: mingo@elte.hu
> CC: akpm@linux-foundation.org
> CC: mingo@elte.hu
> CC: laijs@cn.fujitsu.com
> CC: dipankar@in.ibm.com
> CC: josh@joshtriplett.org
> CC: dvhltc@us.ibm.com
> CC: niv@us.ibm.com
> CC: tglx@linutronix.de
> CC: peterz@infradead.org
> CC: rostedt@goodmis.org
> CC: Valdis.Kletnieks@vt.edu
> CC: dhowells@redhat.com
> CC: eric.dumazet@gmail.com
> CC: Alexey Dobriyan <adobriyan@gmail.com>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> ---
>  include/linux/rcupdate.h |   13 ++++++++++++-
>  include/net/dst.h        |    2 ++
>  kernel/rcutiny.c         |    9 +++++++++
>  kernel/rcutree.c         |   10 ++++++++++
>  lib/Kconfig.debug        |    9 +++++++++
>  5 files changed, 42 insertions(+), 1 deletion(-)
> 
> Index: linux-2.6-lttng/include/linux/rcupdate.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/linux/rcupdate.h	2010-03-18 20:27:25.000000000 -0400
> +++ linux-2.6-lttng/include/linux/rcupdate.h	2010-03-18 20:30:54.000000000 -0400
> @@ -49,6 +49,9 @@
>  struct rcu_head {
>  	struct rcu_head *next;
>  	void (*func)(struct rcu_head *head);
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +	struct rcu_head *debug;
> +#endif
>  };
> 
>  /* Exported common interfaces */
> @@ -71,11 +74,19 @@ extern void rcu_init(void);
>  #error "Unknown RCU implementation specified to kernel configuration"
>  #endif
> 
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +#define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL, .debug = NULL }
> +#define INIT_RCU_HEAD(ptr) do { \
> +       (ptr)->next = NULL; (ptr)->func = NULL; (ptr)->debug = NULL; \
> +} while (0)
> +#else
>  #define RCU_HEAD_INIT	{ .next = NULL, .func = NULL }
> -#define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
>  #define INIT_RCU_HEAD(ptr) do { \
>         (ptr)->next = NULL; (ptr)->func = NULL; \
>  } while (0)
> +#endif
> +
> +#define RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
> 
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
>  extern struct lockdep_map rcu_lock_map;
> Index: linux-2.6-lttng/kernel/rcutree.c
> ===================================================================
> --- linux-2.6-lttng.orig/kernel/rcutree.c	2010-03-18 20:27:25.000000000 -0400
> +++ linux-2.6-lttng/kernel/rcutree.c	2010-03-18 20:31:16.000000000 -0400
> @@ -39,6 +39,7 @@
>  #include <asm/atomic.h>
>  #include <linux/bitops.h>
>  #include <linux/module.h>
> +#include <linux/poison.h>
>  #include <linux/completion.h>
>  #include <linux/moduleparam.h>
>  #include <linux/percpu.h>
> @@ -1060,6 +1061,10 @@ static void rcu_do_batch(struct rcu_stat
>  		next = list->next;
>  		prefetch(next);
>  		trace_rcu_tree_callback(list);
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +		WARN_ON_ONCE(list->debug != LIST_POISON1);
> +		list->debug = NULL;
> +#endif
>  		list->func(list);
>  		list = next;
>  		if (++count >= rdp->blimit)
> @@ -1350,6 +1355,11 @@ __call_rcu(struct rcu_head *head, void (
>  	unsigned long flags;
>  	struct rcu_data *rdp;
> 
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +	WARN_ON_ONCE(head->debug);
> +	head->debug = LIST_POISON1;
> +#endif
> +
>  	head->func = func;
>  	head->next = NULL;
> 
> Index: linux-2.6-lttng/lib/Kconfig.debug
> ===================================================================
> --- linux-2.6-lttng.orig/lib/Kconfig.debug	2010-03-18 20:27:25.000000000 -0400
> +++ linux-2.6-lttng/lib/Kconfig.debug	2010-03-18 20:27:38.000000000 -0400
> @@ -661,6 +661,15 @@ config DEBUG_LIST
> 
>  	  If unsure, say N.
> 
> +config DEBUG_RCU_HEAD
> +	bool "Debug RCU callbacks"
> +	depends on DEBUG_KERNEL
> +	depends on TREE_RCU
> +	help
> +	  Enable this to turn on debugging of RCU list heads (call_rcu() usage).
> +	  Seems to find problems more quickly with stress-tests in single-cpu
> +	  mode.
> +
>  config DEBUG_SG
>  	bool "Debug SG table operations"
>  	depends on DEBUG_KERNEL
> Index: linux-2.6-lttng/include/net/dst.h
> ===================================================================
> --- linux-2.6-lttng.orig/include/net/dst.h	2010-03-18 20:27:25.000000000 -0400
> +++ linux-2.6-lttng/include/net/dst.h	2010-03-18 20:35:02.000000000 -0400
> @@ -159,7 +159,9 @@ static inline void dst_hold(struct dst_e
>  	 * If your kernel compilation stops here, please check
>  	 * __pad_to_align_refcnt declaration in struct dst_entry
>  	 */
> +#ifndef CONFIG_DEBUG_RCU_HEAD
>  	BUILD_BUG_ON(offsetof(struct dst_entry, __refcnt) & 63);
> +#endif
>  	atomic_inc(&dst->__refcnt);
>  }
> 
> Index: linux-2.6-lttng/kernel/rcutiny.c
> ===================================================================
> --- linux-2.6-lttng.orig/kernel/rcutiny.c	2010-03-18 20:35:14.000000000 -0400
> +++ linux-2.6-lttng/kernel/rcutiny.c	2010-03-18 20:39:12.000000000 -0400
> @@ -35,6 +35,7 @@
>  #include <linux/init.h>
>  #include <linux/time.h>
>  #include <linux/cpu.h>
> +#include <linux/poison.h>
> 
>  /* Global control variables for rcupdate callback mechanism. */
>  struct rcu_ctrlblk {
> @@ -163,6 +164,10 @@ static void __rcu_process_callbacks(stru
>  	while (list) {
>  		next = list->next;
>  		prefetch(next);
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +		WARN_ON_ONCE(list->debug != LIST_POISON1);
> +		list->debug = NULL;
> +#endif
>  		list->func(list);
>  		list = next;
>  	}
> @@ -210,6 +215,10 @@ static void __call_rcu(struct rcu_head *
>  {
>  	unsigned long flags;
> 
> +#ifdef CONFIG_DEBUG_RCU_HEAD
> +	WARN_ON_ONCE(head->debug);
> +	head->debug = LIST_POISON1;
> +#endif
>  	head->func = func;
>  	head->next = NULL;
> 
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com