Message-ID: <20070922175958.GD11123@linux.vnet.ibm.com>
Date:	Sat, 22 Sep 2007 10:59:58 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC PATCH] Add CRC checksum for RCU lists

On Fri, Sep 21, 2007 at 05:32:08PM -0400, Steven Rostedt wrote:
> 
> --
> On Fri, 21 Sep 2007, Paul E. McKenney wrote:
> 
> > On Thu, Sep 20, 2007 at 02:34:11PM -0400, Steven Rostedt wrote:
> > > In recent development of the RT kernel, some of our experimental code
> > > corrupted the rcu header. But the side effect (crashing) didn't rear its
> > > ugly head until way after the fact. Discussing this with Paul, he
> > > suggested that RCU should have a "self checking" mechanism to detect
> > > these kind of issues. He also suggested putting in a CRC into the
> > > rcu_head structure.
> > >
> > > This patch does so.
> >
> > Very cool!!!
> 
> Thanks :-)
> 
> > > This patch also takes care to update the crc when rcu_head items are
> > > moved from list to list and whenever the next pointer is modified due to
> > > addition.
> >
> > We can only be thankful that it is not possible to cancel outstanding
> > RCU callbacks...
> 
> true
> 
> > Looks good -- a few suggestions for better fault coverage interspersed
> > below.  But would be useful as is.  (And it would be good to apply after
> > the preempt/boost patches, which are currently undergoing integration
> > with the CPU hotplug patches -- the good news is that this integration
> > eliminates the need for patch #4!)
> >
> > Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Thanks, but this is still going to go through changes, from your comments
> as well as your latest patches.

Good point...

> > > Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> > >
> > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > index fe17d7d..baca7e6 100644
> > > --- a/include/linux/rcupdate.h
> > > +++ b/include/linux/rcupdate.h
> > > @@ -50,13 +50,81 @@
> > >  struct rcu_head {
> > >  	struct rcu_head *next;
> > >  	void (*func)(struct rcu_head *head);
> > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
> > > +	/*
> > > +	 * Checks are made in C files knowing that "next" is
> > > +	 * the first element. So keep it the first element.
> > > +	 */
> > > +	unsigned long crc;
> > > +	void *caller;
> > > +#endif
> > >  };
> >
> > Looks good, but one question -- why not include the caller in the CRC?
> > Not a big deal either way, but would catch a few more cases of corruption.
> > Also, as things stand, the caller pointer can be silently corrupted,
> > which might causes confusion if someone had to examine the RCU callback
> > lists from a crash dump.
> 
> One reason was that the caller was an addition. I originally didn't have
> it, but during my tests, the output was basically useless. It didn't give
> any hint to where things went wrong, so I added it. The CRC is to check
> the rcu is working, not really the checker itself.
> 
> Note, it helped us out lately with Peter's latest file_table patches in
> -rt. With this patch applied, it triggered corruption. That was due to the
> union in the fs.h between the llist and rcu_head there was a dependency
> in the llist where the rcu_head would not grow. The llist can still do a
> spin lock on the lock _after_ the item was added to the call_rcu. Because
> of that union, the locking of the lock corrupted the crc. Set it to one.
> 
> You'll see patches from Peter soon to get rid of that dependency.

Look forward to seeing them!

> > Interchanging the order of "crc" and "caller" would change the strategy,
> > if I understand correctly.
> 
> yep.
> 
> >
> > > -#define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL }
> > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
> > > +
> > > +#define RCU_CRC_MAGIC 0xC4809168UL
> >
> > Very magic indeed -- Google doesn't find it, other than in your
> > patch.  ;-)
> 
> Paul, I'm disappointed in you. That number doesn't ring a bell at all??
> 
> (hint, ignore the 'C' that was added by me).

Well, once Kyle spilled the beans, it was pretty obvious.  ;-)

Might well be a first!!!

> > > +static inline unsigned long rcu_crc_calc(struct rcu_head *head)
> > > +{
> > > +	unsigned int *p = (unsigned int*)head; /* 32 bit */
> > > +	unsigned long crc = RCU_CRC_MAGIC;
> > > +
> > > +	for (p=(void *)head; (void*)p < (void*)&head->crc; p++)
> > > +		crc ^= *p;
> > > +	return crc;
> > > +}
> >
> > Why initialize "p" twice?  (Once in the declaration, and once in the
> > "for" loop, but with different casts.)
> 
> Why? probably because I was half asleep when writing it ;-)
> Will fix.
> 
> > > +static inline void rcu_crc_check(struct rcu_head *head)
> > > +{
> > > +	static int once;
> > > +	if (unlikely(head->crc != rcu_crc_calc(head)) && !once) {
> > > +		once++;
> >
> > Do we want exactly one (give or take concurrent checks), or do we want
> > the first ten or so?  Sometimes having a modest sample rather than
> > exactly one is helpful.
> 
> I added that because during testing, it would flood the serial console. I
> can modify this to whatever deems fit.  Perhaps more hits will give a
> better clue to what went wrong. I could also just add a print_ratelimit to
> it too.

Agreed -- using the already-defined print_ratelimit() sounds much better
than reinventing the wheel.

> > And I know that it doesn't matter in this case, but I cannot stop myself
> > from pointing out the possibility of making "once" be an atomic_t
> > that is initialized to (say) -10, then making the !once check be an
> > atomic_add_return().  (Whew!  Good to get that off my chest!!!)
> 
> Would you prefer the above or the print_ratelimit? Maybe 10 would be best.
> I really don't have a preference.

Definitely print_ratelimit()!!!

[ . . . ]

> > > +# define RCU_CRC_INIT , .crc = RCU_CRC_MAGIC
> >
> > Would need to initialize caller here if you add it to the CRC.
> >
> > > +# define RCU_CRC_SET(ptr) (ptr)->crc = RCU_CRC_MAGIC
> >
> > And here as well.
> >
> > But this has the effect of causing the CRC to be born correct.  Do we
> > really want that?  Suppose someone incorrectly re-initialized a callback
> > that was still on a list.  Wouldn't it be better to get a CRC warning than
> > a NULL pointer exception?  So suggest something like RCU_CRC_MAGIC+1 --
> > perhaps with a RCU_CRC_BAD_MAGIC symbol.
> 
> Yeah, I can scrap those initialization macros. That came about my first
> attempt where I forgot to add the assignment to the call_rcu and it
> obviously failed.  So I added these macros, and it stilled failed. Then I
> saw the mistake I made, fixed it, and it worked. But I never removed these
> macros.  I think we can just keep the crc as zero. That would also fail
> the test. Hmm, maybe we should add a BAD_CRC number so that it will give
> us a hint that something was initiazed incorrectly.

Or maybe a wrapper around the rcu_assign_crc() function that ensures
that the CRC is wrong?

> > > +#else
> > > +# define rcu_crc_calc(head) 0
> > > +# define rcu_crc_check(head) do { } while(0)
> > > +# define rcu_assign_crc(head) do { } while(0)
> > > +# define rcu_check_list(head) do { } while(0)
> > > +# define RCU_CRC_INIT
> > > +# define RCU_CRC_SET(ptr) do { } while(0)
> > > +#endif /* CONFIG_RCU_CRC_HEADER_CHECK */
> > > +
> > > +#define RCU_HEAD_INIT 	{ .next = NULL, .func = NULL RCU_CRC_INIT }
> > >  #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)
> > > +#define INIT_RCU_HEAD(ptr) do {				\
> > > +		(ptr)->next = NULL; (ptr)->func = NULL; \
> > > +		RCU_CRC_SET(ptr);			\
> > > +	} while (0)
> > >
> > >
> > >
> > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> > > index 2c2dd84..4c3cc9c 100644
> > > --- a/kernel/rcupdate.c
> > > +++ b/kernel/rcupdate.c
> > > @@ -76,6 +76,23 @@ static atomic_t rcu_barrier_cpu_count;
> > >  static DEFINE_MUTEX(rcu_barrier_mutex);
> > >  static struct completion rcu_barrier_completion;
> > >
> > > +#ifdef CONFIG_RCU_CRC_HEADER_CHECK
> > > +#define rcu_check_rdp(rdp)			\
> > > +	do {					\
> > > +		rcu_check_list(rdp->nxtlist);	\
> > > +		rcu_check_list(rdp->curlist);	\
> > > +		rcu_check_list(rdp->donelist);	\
> > > +	} while(0)
> > > +#define rcu_crc_update_tail(rdp, tail, list)				\
> > > +	do {								\
> > > +		if ((rdp)->tail != &(rdp)->list)			\
> > > +			rcu_assign_crc((struct rcu_head*)(rdp)->tail);	\
> > > +	} while(0)
> > > +#else
> > > +# define rcu_check_rdp(rdp) do { } while(0)
> > > +# define rcu_crc_update_tail(rdp, tail, list) do { } while(0)
> > > +#endif
> > > +
> > >  #ifdef CONFIG_SMP
> > >  static void force_quiescent_state(struct rcu_data *rdp,
> > >  			struct rcu_ctrlblk *rcp)
> > > @@ -122,14 +139,19 @@ void fastcall call_rcu(struct rcu_head *head,
> > >
> > >  	head->func = func;
> > >  	head->next = NULL;
> > > +	rcu_assign_crc(head);
> > >  	local_irq_save(flags);
> > >  	rdp = &__get_cpu_var(rcu_data);
> > >  	*rdp->nxttail = head;
> >
> > The CRC of the tail element is incorrect at this point, but that is OK
> > because we have interrupts disabled and no other CPU can access our list
> > in the meantime.
> 
> Yep, that was planned.
> 
> >
> > > +	rcu_crc_update_tail(rdp, nxttail, nxtlist);
> > >  	rdp->nxttail = &head->next;
> > >  	if (unlikely(++rdp->qlen > qhimark)) {
> > >  		rdp->blimit = INT_MAX;
> > >  		force_quiescent_state(rdp, &rcu_ctrlblk);
> > >  	}
> > > +
> > > +	rcu_check_rdp(rdp);
> >
> > This check is OK -- no other CPU should be able to manipulate our
> > rdp, and we have interrupts disabled.  Same situation for call_rcu_bh()
> > below.
> >
> > >  	local_irq_restore(flags);
> > >  }
> > >
> > > @@ -157,9 +179,11 @@ void fastcall call_rcu_bh(struct rcu_head *head,
> > >
> > >  	head->func = func;
> > >  	head->next = NULL;
> > > +	rcu_assign_crc(head);
> > >  	local_irq_save(flags);
> > >  	rdp = &__get_cpu_var(rcu_bh_data);
> > >  	*rdp->nxttail = head;
> > > +	rcu_crc_update_tail(rdp, nxttail, nxtlist);
> > >  	rdp->nxttail = &head->next;
> > >
> > >  	if (unlikely(++rdp->qlen > qhimark)) {
> > > @@ -167,6 +191,8 @@ void fastcall call_rcu_bh(struct rcu_head *head,
> > >  		force_quiescent_state(rdp, &rcu_bh_ctrlblk);
> > >  	}
> > >
> > > +	rcu_check_rdp(rdp);
> > > +
> > >  	local_irq_restore(flags);
> > >  }
> > >
> > > @@ -233,6 +259,8 @@ static void rcu_do_batch(struct rcu_data *rdp)
> > >  	struct rcu_head *next, *list;
> > >  	int count = 0;
> > >
> > > +	rcu_check_rdp(rdp);
> >
> > OK, interrupts disabled here.
> >
> > >  	list = rdp->donelist;
> > >  	while (list) {
> > >  		next = list->next;
> >
> > Why not invalidate the CRC of the element that we just removed?  This
> > would catch some cases of list mangling.
> 
> Good idea, will add.
> 
> >
> > > @@ -373,6 +401,7 @@ static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
> > >  {
> > >  	local_irq_disable();
> > >  	*this_rdp->nxttail = list;
> >
> > Momentarily wrong CRC OK, interrupts disabled here.  Ditto for
> > __rcu_process_callbacks() below.
> 
> yep
> 
> >
> > > +	rcu_crc_update_tail(this_rdp, nxttail, nxtlist);
> > >  	if (list)
> > >  		this_rdp->nxttail = tail;
> > >  	local_irq_enable();
> > > @@ -424,6 +453,7 @@ static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
> > >  {
> > >  	if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
> > >  		*rdp->donetail = rdp->curlist;
> > > +		rcu_crc_update_tail(rdp, donetail, donelist);
> > >  		rdp->donetail = rdp->curtail;
> > >  		rdp->curlist = NULL;
> > >  		rdp->curtail = &rdp->curlist;
> > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > > index 50a94ee..981fc93 100644
> > > --- a/lib/Kconfig.debug
> > > +++ b/lib/Kconfig.debug
> > > @@ -424,6 +424,17 @@ config RCU_TORTURE_TEST
> > >  	  Say M if you want the RCU torture tests to build as a module.
> > >  	  Say N if you are unsure.
> > >
> > > +config RCU_CRC_HEADER_CHECK
> > > +	bool "RCU header self check"
> > > +	depends on DEBUG_KERNEL
> > > +	help
> > > +	  This option enables CRC verification of RCU lists to catch
> > > +	  possible corruption to the RCU list by improper application
> > > +	  of RCU callbacks. This adds overhead to the running system
> > > +	  so only enable it if you suspect RCU corruption is occurring.
> > > +
> > > +	  If unsure, say N.
> > > +
> > >  config LKDTM
> > >  	tristate "Linux Kernel Dump Test Tool Module"
> > >  	depends on DEBUG_KERNEL
> > >
> > >
> >
> 
> Paul, thanks for all the comments. I'll put out a new round of patches
> after yours becomes offical (no "not for inclussion" statements).

Sounds good!!!

						Thanx, Paul
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