Subject: Re: [PATCH 2/2] ipmr: Add lockdep expression to ipmr_for_each_table macro

From: Amol Grover <[email-masked]>
Date: Tue, 25 Feb 2020 18:40:41 +0530

On Mon, Feb 24, 2020 at 01:18:01PM -0800, David Miller wrote:
> From: Amol Grover <[email-masked]>
> Date: Sat, 22 Feb 2020 12:08:36 +0530
> 
> > ipmr_for_each_table() uses list_for_each_entry_rcu() for
> > traversing outside of an RCU read-side critical section but
> > under the protection of pernet_ops_rwsem. Hence add the
> > corresponding lockdep expression to silence the following
> > false-positive warning at boot:
> > 
> > [    0.645292] =============================
> > [    0.645294] WARNING: suspicious RCU usage
> > [    0.645296] 5.5.4-stable #17 Not tainted
> > [    0.645297] -----------------------------
> > [    0.645299] net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
> > 
> > Signed-off-by: Amol Grover <[email-masked]>
> 
> This patch series causes build problems, please fix and resubmit the entire
> series:
> 
> [davem@localhost net-next]$ make net/ipv4/ipmr.o 
>   CALL    scripts/checksyscalls.sh
>   CALL    scripts/atomic/check-atomics.sh
>   DESCEND  objtool
>   CC      net/ipv4/ipmr.o
> In file included from ./include/linux/rculist.h:11,
>                  from ./include/linux/pid.h:5,
>                  from ./include/linux/sched.h:14,
>                  from ./include/linux/uaccess.h:5,
>                  from net/ipv4/ipmr.c:24:
> net/ipv4/ipmr.c: In function ‘ipmr_get_table’:
> ./include/linux/rculist.h:63:25: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
>   RCU_LOCKDEP_WARN(!cond && !rcu_read_lock_any_held(),  \
> ./include/linux/rcupdate.h:263:52: note: in definition of macro ‘RCU_LOCKDEP_WARN’
>    if (debug_lockdep_rcu_enabled() && !__warned && (c)) { \
>                                                     ^
> ./include/linux/rculist.h:381:7: note: in expansion of macro ‘__list_check_rcu’
>   for (__list_check_rcu(dummy, ## cond, 0),   \
>        ^~~~~~~~~~~~~~~~
> net/ipv4/ipmr.c:113:2: note: in expansion of macro ‘list_for_each_entry_rcu’
>   list_for_each_entry_rcu(mrt, &net->ipv4.mr_tables, list, \
>   ^~~~~~~~~~~~~~~~~~~~~~~
> net/ipv4/ipmr.c:138:2: note: in expansion of macro ‘ipmr_for_each_table’
>   ipmr_for_each_table(mrt, net) {
>   ^~~~~~~~~~~~~~~~~~~

This is a build warning due to incorrect operator precedence in
__list_check_rcu macro. This has already been reported and the patch
submitted [1]. Infact the patch is currently queued for v5.7.

[1] https://lore.kernel.org/patchwork/patch/1181886/

Thanks
Amol