Re: [PATCH] Default enable RCU list lockdep debugging with PROVE_RCU

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Mar 05, 2020 at 11:09:54PM +0530, Madhuparna Bhowmik wrote:
> On Thu, Mar 05, 2020 at 07:52:38AM -0800, Guenter Roeck wrote:
> > On Fri, Feb 28, 2020 at 02:54:51PM +0530, madhuparnabhowmik10@gmail.com wrote:
> > > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > > 
> > > This patch default enables CONFIG_PROVE_RCU_LIST option with
> > > CONFIG_PROVE_RCU for RCU list lockdep debugging.
> > > 
> > > With this change, RCU list lockdep debugging will be default
> > > enabled in CONFIG_PROVE_RCU=y kernels.
> > > 
> > > Most of the RCU users (in core kernel/, drivers/, and net/
> > > subsystem) have already been modified to include lockdep
> > > expressions hence RCU list debugging can be enabled by
> > > default.
> > > 
> > > However, there are still chances of enountering
> > > false-positive lockdep splats because not everything is converted,
> > > in case RCU list primitives are used in non-RCU read-side critical
> > > section but under the protection of a lock. It would be okay to
> > > have a few false-positives, as long as bugs are identified, since this
> > > patch only affects debugging kernels.
> > > 
> > > Co-developed-by: Amol Grover <frextrite@gmail.com>
> > > Signed-off-by: Amol Grover <frextrite@gmail.com>
> > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > 
> > Who is going to fix the fallout ?
> > 
> > fs/btrfs/block-group.c:2011 RCU-list traversed in non-reader section!!
> > kernel/kprobes.c:329 RCU-list traversed in non-reader section!!
> > net/ipv4/ipmr.c:136 RCU-list traversed in non-reader section!!
> >
> Hi,
> There is already a patch for fixing the warnings in kernel/kprobes.c :
> https://lore.kernel.org/lkml/157905963533.2268.4672153983131918123.stgit@devnote2/
> 
> Same for net/ipv4/ipmr:
> https://lore.kernel.org/patchwork/patch/1198934/
> 
> Can you please send the warning with the stack backtrace and locks held
> for btrfs/block-group.c, I will work on it.
> 

See below. I think that should be easy to reproduce by mounting
a btrfs file system.

Guenter

---
[   28.920119] BTRFS: device fsid afe7540f-98fe-4a5c-ba94-3fb85a5da345 devid 1 transid 6 /dev/root scanned by swapper/0 (1)
[   28.961347] BTRFS info (device sda): disk space caching is enabled
[   28.963199] BTRFS info (device sda): has skinny extents
[   28.963427] BTRFS info (device sda): flagging fs with big metadata feature
[   29.104392]
[   29.104591] =============================
[   29.104756] WARNING: suspicious RCU usage
[   29.105046] 5.6.0-rc4-next-20200305 #1 Not tainted
[   29.105231] -----------------------------
[   29.105401] fs/btrfs/block-group.c:2011 RCU-list traversed in non-reader section!!
[   29.105643]
[   29.105643] other info that might help us debug this:
[   29.105643]
[   29.106344]
[   29.106344] rcu_scheduler_active = 2, debug_locks = 1
[   29.106590] 1 lock held by swapper/0/1:
[   29.106776]  #0: ffff0000199e90d8 (&type->s_umount_key#23/1){+.+.}, at: alloc_super+0xac/0x2c0
[   29.107436]
[   29.107436] stack backtrace:
[   29.107784] CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.6.0-rc4-next-20200305 #1
[   29.107989] Hardware name: linux,dummy-virt (DT)
[   29.108344] Call trace:
[   29.108488]  dump_backtrace+0x0/0x1a0
[   29.108638]  show_stack+0x14/0x20
[   29.108784]  dump_stack+0xe8/0x150
[   29.108921]  lockdep_rcu_suspicious+0xf8/0x108
[   29.109071]  btrfs_read_block_groups+0x754/0x860
[   29.109222]  open_ctree+0xe74/0x1580
[   29.109359]  btrfs_mount_root+0x3cc/0x4c0
[   29.109514]  legacy_get_tree+0x2c/0x60
[   29.109653]  vfs_get_tree+0x24/0xe8
[   29.109872]  fc_mount+0x14/0x50
[   29.110010]  vfs_kern_mount.part.41+0x68/0x98
[   29.110158]  vfs_kern_mount+0x10/0x20
[   29.110297]  btrfs_mount+0x158/0x4b0
[   29.110434]  legacy_get_tree+0x2c/0x60
[   29.110573]  vfs_get_tree+0x24/0xe8
[   29.110709]  do_mount+0x568/0x998
[   29.110849]  do_mount_root+0x8c/0x11c
[   29.110990]  mount_block_root+0x114/0x244
[   29.111134]  mount_root+0x124/0x154
[   29.111272]  prepare_namespace+0x128/0x164
[   29.111417]  kernel_init_freeable+0x298/0x2b8
[   29.111569]  kernel_init+0x10/0x100
[   29.111710]  ret_from_fork+0x10/0x18