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Date: Fri, 19 Mar 2010 09:54:51 -0400
From: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: Lai Jiangshan <laijs@...fujitsu.com>, mingo@...e.hu,
akpm@...ux-foundation.org, dipankar@...ibm.com,
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>,
linux-kernel@...r.kernel.org,
Christoph Lameter <cl@...ux-foundation.org>,
Pekka Enberg <penberg@...helsinki.fi>
Subject: Re: [PATCH] tree/tiny rcu: Add debug RCU head option (v2)
* Paul E. McKenney (paulmck@...ux.vnet.ibm.com) wrote:
> On Fri, Mar 19, 2010 at 12:13:53PM +0800, Lai Jiangshan wrote:
> > CC: Christoph Lameter <cl@...ux-foundation.org>
> > CC: Pekka Enberg <penberg@...helsinki.fi>
> >
> > I have written a patch like this before.
> >
> > but SLUB opposed me, so I did not post it.
> >
> > @ slub.c
> > static void free_slab(struct kmem_cache *s, struct page *page)
> > {
> > if (unlikely(s->flags & SLAB_DESTROY_BY_RCU)) {
> > /*
> > * RCU free overloads the RCU head over the LRU
> > */
> > struct rcu_head *head = (void *)&page->lru;
> >
> > call_rcu(head, rcu_free_slab);
> > } else
> > __free_slab(s, page);
> > }
> >
> > I think it is not your patch's fault, but the above code has
> > already existed.
> >
> > In your patch, struct rcu_head::debug will share the same memory of
> > the other field of struct page, (especially share the struct page::flags
> > which I encountered)
> >
> > I tested it very long time before, so I can't give you dmesg.
>
> Ouch!!!
>
> If this only causes a problem for SLUB, maybe a workaround is to make
> the config variable depend on !SLUB. But this would seem to increase
> the urgency of using the debug objects
Hrm.. looking into the port to debugobjects, there are a few things that are not
precisely well set at the moment for it:
- We need to have two distinct active states, with a check that we move between
the two. This does not seem to be currently supported by debugobjects.
- RCU heads are never explicitely freed. So yeah, that means we will have to
modify a lot of callers.
Thanks,
Mathieu
>
> Thanx, Paul
>
> > Lai
> >
> > 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.
> > >
> > > 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
> > > CC: mingo@...e.hu
> > > CC: laijs@...fujitsu.com
> > > CC: dipankar@...ibm.com
> > > CC: josh@...htriplett.org
> > > CC: dvhltc@...ibm.com
> > > CC: niv@...ibm.com
> > > CC: tglx@...utronix.de
> > > CC: peterz@...radead.org
> > > CC: rostedt@...dmis.org
> > > CC: Valdis.Kletnieks@...edu
> > > CC: dhowells@...hat.com
> > > CC: eric.dumazet@...il.com
> > > CC: Alexey Dobriyan <adobriyan@...il.com>
> > > CC: Peter Zijlstra <a.p.zijlstra@...llo.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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