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Date: Thu, 18 Mar 2010 16:22:02 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
dipankar@...ibm.com, akpm@...ux-foundation.org,
josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
Valdis.Kletnieks@...edu, dhowells@...hat.com,
eric.dumazet@...il.com, Alexey Dobriyan <adobriyan@...il.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH tip/core/urgent 2/2] rcu: remove INIT_RCU_HEAD,
RCU_HEAD_INIT, RCU_HEAD
* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> On Thu, Mar 18, 2010 at 03:35:20PM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> > > From: Alexey Dobriyan <adobriyan@...il.com>
> > >
> > > call_rcu() will unconditionally reinitialize RCU head anyway. New users
> > > of these macros constantly appear, so remove them.
> >
> > Hrm. So do we have something that checks for double-use of a RCU head at
> > the moment ? (using call_rcu() twice on the same head without being
> > certain that the first callback have finished its execution).
> >
> > I think that hiding rcu head initialization into call_rcu() is one more
> > step towards misuses that will silently corrupt rcu head lists. So I
> > think we should first add the double-use debugging option before we
> > remove the RCU head initializations.
>
> So your thought is to have rcu_do_batch() do something like the
> following?
>
> ...
>
> next = list->next;
> prefetch(next);
> list->next = RCU_HEAD_INIT_PTR;
> func = list->func;
> list->func = RCU_HEAD_INIT_PTR;
> func(list);
> ... /* touching anything referenced by "list" is use-after-free. */
>
> Then have __call_rcu() do something like the following before initializing
> the ->func and ->next pointers:
>
> WARN_ON_ONCE(head->next != RCU_HEAD_INIT_PTR ||
> head->func != RCU_HEAD_INIT_PTR);
>
> And then require that all users of call_rcu() and friends use one of the
> RCU_INIT() macros?
>
> Or did you have something else in mind?
More precisely poisoning an extra field of the rcu_head, as done in the
following patch.
I posted it a few months ago, but has been rejected on the ground that
it should be re-done in within the debug objects infrastructure. But I
had to focus on other things and never found time to do these changes.
It needs a separate patch which adds missing INIT_RCU_HEAD() to a few
more kernel sites.
The reason why I add a supplementary field for the poison is to be able
to warn for detection of incoherent list_head both in call_rcu and in
rcu_do_batch(), which does not seem possible with the scheme you propose
above. The sequence is:
init -> debug = NULL
call_rcu -> WARN_ON_ONCE(debug != NULL)
debug = LIST_POISON1
rcu_do_batch -> WARN_ON_ONCE(debug != LIST_POISON1)
debug = NULL
tree rcu: Add debug RCU head option
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.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC: mingo@...e.hu
CC: akpm@...ux-foundation.org
---
include/linux/rcupdate.h | 11 +++++++++++
include/net/dst.h | 2 ++
kernel/rcutree.c | 10 ++++++++++
lib/Kconfig.debug | 9 +++++++++
4 files changed, 32 insertions(+)
Index: linux-2.6-lttng/include/linux/rcupdate.h
===================================================================
--- linux-2.6-lttng.orig/include/linux/rcupdate.h 2009-11-22 20:25:49.000000000 -0500
+++ linux-2.6-lttng/include/linux/rcupdate.h 2009-11-22 22:11:48.000000000 -0500
@@ -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 */
@@ -77,11 +80,19 @@ extern int rcu_scheduler_active;
#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 RCU_HEAD(head) struct rcu_head head = RCU_HEAD_INIT
+#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
#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 2009-11-22 21:38:56.000000000 -0500
+++ linux-2.6-lttng/kernel/rcutree.c 2009-11-22 22:10:49.000000000 -0500
@@ -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>
@@ -1010,6 +1011,10 @@ static void rcu_do_batch(struct rcu_stat
next = list->next;
prefetch(next);
trace_rcu_tree_callback(list);
+#ifdef DEBUG_RCU_HEAD
+ WARN_ON_ONCE(list->debug != LIST_POISON1);
+ list->debug = NULL;
+#endif
list->func(list);
list = next;
if (++count >= rdp->blimit)
@@ -1291,6 +1296,11 @@ __call_rcu(struct rcu_head *head, void (
unsigned long flags;
struct rcu_data *rdp;
+#ifdef 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 2009-11-22 22:01:03.000000000 -0500
+++ linux-2.6-lttng/lib/Kconfig.debug 2009-11-22 22:10:49.000000000 -0500
@@ -652,6 +652,15 @@ 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 2009-11-22 20:25:49.000000000 -0500
+++ linux-2.6-lttng/include/net/dst.h 2009-11-22 22:10:49.000000000 -0500
@@ -154,7 +154,9 @@ 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);
}
--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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