Re: next-20191108: qemu arm64: WARNING: suspicious RCU usage

[Paul E. McKenney]

On Mon, Nov 11, 2019 at 08:59:25AM +0100, Anders Roxell wrote:
> Hi,
> 
> I'm seeing the following warning when I'm booting an arm64 allmodconfig
> kernel [1] on linux-next tag next-20191108, is this anything you've seen
> before ?
> 
> 
> The code seems to have introduced that is f0ad0860d01e ("ipv4: ipmr:
> support multiple tables") in 2010 and the warning was added reacently
> 28875945ba98 ("rcu: Add support for consolidated-RCU reader checking").
> 
> 
> [   32.496021][    T1] =============================
> [   32.497616][    T1] WARNING: suspicious RCU usage
> [   32.499614][    T1] 5.4.0-rc6-next-20191108-00003-gf74bac957b5c-dirty #2 Not tainted
> [   32.502018][    T1] -----------------------------
> [   32.503976][    T1] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
> [   32.506746][    T1] 
> [   32.506746][    T1] other info that might help us debug this:
> [   32.506746][    T1] 
> [   32.509794][    T1] 
> [   32.509794][    T1] rcu_scheduler_active = 2, debug_locks = 1
> [   32.512661][    T1] 1 lock held by swapper/0/1:
> [   32.514169][    T1]  #0: ffffa000150dd678 (pernet_ops_rwsem){+.+.}, at: register_pernet_subsys+0x24/0x50
> [   32.517621][    T1] 
> [   32.517621][    T1] stack backtrace:
> [   32.519930][    T1] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc6-next-20191108-00003-gf74bac957b5c-dirty #2
> [   32.523063][    T1] Hardware name: linux,dummy-virt (DT)
> [   32.524787][    T1] Call trace:
> [   32.525946][    T1]  dump_backtrace+0x0/0x2d0
> [   32.527433][    T1]  show_stack+0x20/0x30
> [   32.528811][    T1]  dump_stack+0x204/0x2ac
> [   32.530258][    T1]  lockdep_rcu_suspicious+0xf4/0x108
> [   32.531993][    T1]  ipmr_get_table+0xc8/0x170

So this one is invoking ipmr_for_each_table(), which in turn invokes
list_for_each_entry_rcu(), which really does want to be in an
RCU read-side critical section.  (But you can pass it an optional
additional lockdep expressions.

> [   32.533496][    T1]  ipmr_new_table+0x48/0xa0

And this does look like update-side code...

> [   32.535002][    T1]  ipmr_net_init+0xe8/0x258

And this one is marked with "__net_init", which turns out to be __init.
So this is being invoked during early boot (see inet_init() below).
Or with RTNL held when invoked at runtime.  So, can we make a lockdep
expression for this combination?

The RTNL part is easy, something like this in include/linux/rtnetlink.h:

#ifdef CONFIG_PROVE_LOCKING
extern int lockdep_rtnl_is_held(void);
#else
#define lockdep_rtnl_is_held() 1
#endif

And in net/core/rtnetlink.c:

#ifdef CONFIG_PROVE_LOCKING
int lockdep_rtnl_is_held(void)
{
	return lockdep_is_held(&rtnl_mutex);
}
#endif

> [   32.536465][    T1]  ops_init+0x280/0x2d8
> [   32.537876][    T1]  register_pernet_operations+0x210/0x420
> [   32.539707][    T1]  register_pernet_subsys+0x30/0x50
> [   32.541372][    T1]  ip_mr_init+0x54/0x180
> [   32.542785][    T1]  inet_init+0x25c/0x3e8

And this is an fs_initcall().  This is late enough during boot that
RTNL could conceivably be held, but I don't see evidence of that.
One approach would be to hold RTNL across this initialization code.

So the other approach would be to have a global variable in net/ipv4/ipmr.c
whose definition depends on whether lockdep is enabled:

#ifdef CONFIG_PROVE_LOCKING
int ip_mr_initialized;
void ip_mr_now_initialized(void) { ip_mr_initialized = 1; }
#else
const int ip_mr_initialized = 1;
void ip_mr_now_initialized(void) { }
#endif

Then at the end of ip_mr_init():

	ip_mr_now_initialized();

And finally change the CONFIG_IP_MROUTE_MULTIPLE_TABLES definition
of ipmr_for_each_table() to be something like:

#define ipmr_for_each_table(mrt, net) \
	list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
				lockdep_rtnl_is_held() || !ip_mr_initialized)

> [   32.544186][    T1]  do_one_initcall+0x4c0/0xad8
> [   32.545757][    T1]  kernel_init_freeable+0x3e0/0x500
> [   32.547443][    T1]  kernel_init+0x14/0x1f0
> [   32.548875][    T1]  ret_from_fork+0x10/0x18

Does that work for you?

							Thanx, Paul