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

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


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