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

[Anders Roxell]

On Mon, 11 Nov 2019 at 14:34, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> 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?

Yes, that made the "suspicious RCU usage" warning go away.

Cheers,
Anders