Subject: Re: [PATCH] Fix RCU warning in rt_cache_seq_show

From: "Paul E. McKenney" <[email-masked]>
Date: Fri, 12 Aug 2011 05:49:59 -0700

On Fri, Aug 12, 2011 at 09:23:50AM +0200, Eric Dumazet wrote:
> Le jeudi 11 août 2011 à 19:32 -0700, Paul E. McKenney a écrit :
> > On Thu, Aug 11, 2011 at 06:58:21PM +0200, Eric Dumazet wrote:
> > > Le mercredi 10 août 2011 à 10:28 +0100, Mark Rutland a écrit :
> > > > > -----Original Message-----
> > > > > From: Eric Dumazet [mailto:[email-masked]]
> > > > > Sent: 09 August 2011 18:19
> > > > > To: Mark Rutland; Paul E. McKenney
> > > > > Cc: [email-masked]; David S. Miller; Gergely Kalman
> > > > > Subject: Re: [PATCH] Fix RCU warning in rt_cache_seq_show
> > > > > 
> > > > > Le mardi 09 août 2011 à 18:02 +0100, Mark Rutland a écrit :
> > > > > > Commit f2c31e32 ("net: fix NULL dereferences in check_peer_redir()")
> > > > > > added rcu protection to dst neighbour, and updated callsites for
> > > > > > dst_{get,set}_neighbour. Unfortunately, it missed rt_cache_seq_show.
> > > > > >
> > > > > > This produces a warning on v3.1-rc1 (on a preemptible kernel, on an
> > > > > > ARM Vexpress A9x4):
> > > > > >
> > > > > > ===================================================
> > > > > > [ INFO: suspicious rcu_dereference_check() usage. ]
> > > > > > ---------------------------------------------------
> > > > > > include/net/dst.h:91 invoked rcu_dereference_check() without
> > > > > protection!
> > > > > >
> > > > > > other info that might help us debug this:
> > > > > >
> > > > > > rcu_scheduler_active = 1, debug_locks = 0
> > > > > > 2 locks held by proc01/32159:
> > > > > >
> > > > > > stack backtrace:
> > > > > > [<80014880>] (unwind_backtrace+0x0/0xf8) from [<802e5c78>]
> > > > > (rt_cache_seq_show+0x18c/0x1c4)
> > > > > > [<802e5c78>] (rt_cache_seq_show+0x18c/0x1c4) from [<800e0c5c>]
> > > > > (seq_read+0x324/0x4a4)
> > > > > > [<800e0c5c>] (seq_read+0x324/0x4a4) from [<8010786c>]
> > > > > (proc_reg_read+0x70/0x94)
> > > > > > [<8010786c>] (proc_reg_read+0x70/0x94) from [<800c0ba8>]
> > > > > (vfs_read+0xb0/0x144)
> > > > > > [<800c0ba8>] (vfs_read+0xb0/0x144) from [<800c0ea8>]
> > > > > (sys_read+0x40/0x70)
> > > > > > [<800c0ea8>] (sys_read+0x40/0x70) from [<8000e0c0>]
> > > > > (ret_fast_syscall+0x0/0x3c)
> > > > > >
> > > > > > This patch adds calls to rcu_read_{lock,unlock} in rt_cache_seq_show,
> > > > > > protecting the dereferenced variable, and clearing the warning.
> > > > > >
> > > > > > Signed-off-by: Mark Rutland <[email-masked]>
> > > > > > Cc: David S. Miller <[email-masked]>
> > > > > > Cc: Eric Dumazet <[email-masked]>
> > > > > > Cc: Gergely Kalman <[email-masked]>
> > > > > > ---
> > > > > >  net/ipv4/route.c |    2 ++
> > > > > >  1 files changed, 2 insertions(+), 0 deletions(-)
> > > > > >
> > > > > > diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> > > > > > index e3dec1c..6699ef7 100644
> > > > > > --- a/net/ipv4/route.c
> > > > > > +++ b/net/ipv4/route.c
> > > > > > @@ -419,6 +419,7 @@ static int rt_cache_seq_show(struct seq_file
> > > > > *seq, void *v)
> > > > > >  		struct neighbour *n;
> > > > > >  		int len;
> > > > > >
> > > > > > +		rcu_read_lock();
> > > > > >  		n = dst_get_neighbour(&r->dst);
> > > > > >  		seq_printf(seq, "%s\t%08X\t%08X\t%8X\t%d\t%u\t%d\t"
> > > > > >  			      "%08X\t%d\t%u\t%u\t%02X\t%d\t%1d\t%08X%n",
> > > > > > @@ -435,6 +436,7 @@ static int rt_cache_seq_show(struct seq_file
> > > > > *seq, void *v)
> > > > > >  			-1,
> > > > > >  			(n && (n->nud_state & NUD_CONNECTED)) ? 1 : 0,
> > > > > >  			r->rt_spec_dst, &len);
> > > > > > +		rcu_read_unlock();
> > > > > >
> > > > > >  		seq_printf(seq, "%*s\n", 127 - len, "");
> > > > > >  	}
> > > > > 
> > > > > 
> > > > > Hmm, I though rcu_read_lock_bh() (done by caller of this function) was
> > > > > protecting us here.
> > > > 
> > > > Aha. Being a bit trigger-happy, I'd had a quick look at the functions
> > > > mentioned in the backtrace, and not looked at any possible inlining.
> > > > 
> > > > This being my first real exposure to RCU, I wasn't aware of the *_bh
> > > > variants. Looking at the documentation (Documentation/RCU/checklist.txt),
> > > > I think the real problem is that we should be using rcu_dereference_bh in
> > > > this case:
> > > > 
> > > >   > read-side critical sections are delimited by rcu_read_lock()
> > > >   > and rcu_read_unlock(), or by similar primitives such as
> > > >   > rcu_read_lock_bh() and rcu_read_unlock_bh(), in which case
> > > >   > the matching rcu_dereference() primitive must be used in order
> > > >   > to keep lockdep happy, in this case, rcu_dereference_bh().
> > > 
> > > Hmm.
> > > 
> > > I do think dst_get_neighbour() should use rcu_dereference(), because
> > > dst->_neighbour are freed by call_rcu().
> > > 
> > > The question is : Is following construct [A] safe or not ?
> > > 
> > > {
> > > rcu_read_lock_bh();
> > > 	/* BH are now disabled, and we are not allowed to sleep */
> > > 	...
> > > 
> > > 	ptr = rcu_dereference();
> > 
> > This should be:
> > 
> > 	ptr = rcu_dereference_bh();
> > 
> > As you say below.  Never mind!  ;-)
> > 
> > > 	...
> > > rcu_read_unlock_bh();
> > > }
> > > 
> > > 
> > > I dont really understand why lockdep wants [B] instead :
> > > 
> > > {
> > > rcu_read_lock_bh();
> > > 	...
> > > 
> > > 	{
> > > 	rcu_read_lock();
> > > 	ptr = rcu_dereference();
> > 
> > Here you are protected by both RCU and RCU-bh, so you should be able
> > to use either rcu_dereference() or rcu_dereference_bh().  A bit
> > strange to use rcu_dereference_bh(), though.  Except perhaps if a
> > pointer to a function was passed in from the outer RCU-bh read-side
> > critical section or something.
> > 
> > > 	rcu_read_unlock();
> > > 	}
> > > 	...
> > > rcu_read_unlock_bh();
> > > }
> > > 
> > > 
> > > 
> > > However, I can understand the other way [C], this is really needed :
> > > 
> > > {
> > > rcu_read_lock();
> > > 	...
> > > 
> > > 	{
> > > 	rcu_read_lock_bh();
> > > 	ptr = rcu_dereference_bh();
> > > 	rcu_read_unlock_bh();
> > > 	}
> > > 	...
> > > rcu_read_unlock();
> > > }
> > > 
> > > I believe [A] should be allowed by lockdep.
> > 
> > OK, I'll bite.  Why?
> > 
> 
> Oh well, I assumed local_bh_disable() disables preemption.
> 
> It does since day-0
> add_preempt_count(SOFTIRQ_DISABLE_OFFSET);
> 
> So following should be safe :
> 
> local_bh_disable();
> {
> ptr = rcu_dereference(...);
> use(ptr);
> }
> local_bh_enable();
> 
> Maybe they are longterm plans to break this assumption, I dont know.

It would be safe for TINY_RCU and TREE_RCU, but not for either
TINY_PREEMPT_RCU or TREE_PREEMPT_RCU.  These last two do not
recognize a preempt-disable region as an RCU read-side critical
section.

						Thanx, Paul