Messages in this thread | | | Date | Thu, 18 Mar 2010 14:04:32 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH tip/core/urgent 2/2] rcu: remove INIT_RCU_HEAD, RCU_HEAD_INIT, RCU_HEAD |
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On Thu, Mar 18, 2010 at 04:22:02PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > On Thu, Mar 18, 2010 at 03:35:20PM -0400, Mathieu Desnoyers wrote: > > > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > > > From: Alexey Dobriyan <adobriyan@gmail.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.
Indeed!
> 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.
This does look attractive!
Some comments below.
> 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 > --- > 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
RCU_HEAD() is identical in either case, so should be pulled out of the #ifdef, right?
> #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
This needs to be CONFIG_DEBUG_RCU_HEAD, right?
> + 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
Ditto here.
> + 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
You lost me on this one.
> atomic_inc(&dst->__refcnt); > }
Would you be willing to add this to TINY_RCU as well? It would be under #ifdef, so would not affect the size of production builds.
Thanx, Paul
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