> 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.
>
> 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;
>
>
