Poisoning the rcu_head callback list. Only for rcu tree for now. Helps finding racy users of call_rcu(), which results in hangs because list entries are overwritten and/or skipped. Using the lower bit to poison because include/net/dst.h __pad_to_align_refcnt complains when struct rcu_head grows. Signed-off-by: Mathieu Desnoyers CC: "Paul E. McKenney" CC: mingo@elte.hu CC: akpm@linux-foundation.org --- include/linux/rcupdate.h | 2 ++ kernel/rcutree.c | 10 ++++++++++ lib/Kconfig.debug | 9 +++++++++ 3 files changed, 21 insertions(+) Index: linux-2.6-lttng/include/linux/rcupdate.h =================================================================== --- linux-2.6-lttng.orig/include/linux/rcupdate.h 2009-10-06 10:35:15.000000000 -0400 +++ linux-2.6-lttng/include/linux/rcupdate.h 2009-10-06 10:35:18.000000000 -0400 @@ -45,6 +45,8 @@ * struct rcu_head - callback structure for use with RCU * @next: next update requests in a list * @func: actual update function to call after the grace period. + * + * Debug mode assumes func pointer value is word-aligned. */ struct rcu_head { struct rcu_head *next; Index: linux-2.6-lttng/kernel/rcutree.c =================================================================== --- linux-2.6-lttng.orig/kernel/rcutree.c 2009-10-06 10:35:15.000000000 -0400 +++ linux-2.6-lttng/kernel/rcutree.c 2009-10-06 10:35:27.000000000 -0400 @@ -927,6 +927,10 @@ static void rcu_do_batch(struct rcu_data next = list->next; prefetch(next); trace_rcu_tree_callback(list); +#ifdef DEBUG_RCU_HEAD + WARN_ON_ONCE(!((unsigned long)list->func & 0x1)); + list->func = (void *)((unsigned long)list->func & ~0x1); +#endif list->func(list); list = next; if (++count >= rdp->blimit) @@ -1194,7 +1198,13 @@ __call_rcu(struct rcu_head *head, void ( unsigned long flags; struct rcu_data *rdp; +#ifdef DEBUG_RCU_HEAD + WARN_ON_ONCE((unsigned long)head->func & 0x1); + WARN_ON_ONCE((unsigned long)func & 0x1); + head->func = (void *)((unsigned long)func | 0x1); +#else head->func = func; +#endif head->next = NULL; smp_mb(); /* Ensure RCU update seen before callback registry. */ Index: linux-2.6-lttng/lib/Kconfig.debug =================================================================== --- linux-2.6-lttng.orig/lib/Kconfig.debug 2009-10-06 10:35:15.000000000 -0400 +++ linux-2.6-lttng/lib/Kconfig.debug 2009-10-06 10:35:18.000000000 -0400 @@ -598,6 +598,15 @@ 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 -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/