Re: [PATCH] fs: btrfs: block-group.c: Fix suspicious RCU usage warning

[David Sterba]

On Fri, Mar 06, 2020 at 07:30:24PM +0530, Madhuparna Bhowmik wrote:
> On Fri, Mar 06, 2020 at 03:16:53PM +0800, Qu Wenruo wrote:
> > 
> > 
> > On 2020/3/6 äå2:52, madhuparnabhowmik10@xxxxxxxxx wrote:
> > > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > > 
> > > The space_info list is rcu protected.
> > > Hence, it should be traversed with rcu_read_lock held.
> > > 
> > > Warning:
> > > [   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!!
> > > 
> > > Reported-by: Guenter Roeck <linux@xxxxxxxxxxxx>
> > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx>
> > > ---
> > >  fs/btrfs/block-group.c | 4 +++-
> > >  1 file changed, 3 insertions(+), 1 deletion(-)
> > > 
> > > diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c
> > > index 404e050ce8ee..9cabeef66f5b 100644
> > > --- a/fs/btrfs/block-group.c
> > > +++ b/fs/btrfs/block-group.c
> > > @@ -1987,6 +1987,7 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> > 
> > This function is only triggered at mount time, where no other rcu
> > operation can happen.
> >
> Thanks Qu.
> 
> Joel and Paul, what should we do in this case?
> Should we just pass cond = true or use list_for_each_entry instead?

I think we can afford to add rcu lock/unlock, even if it's not strictly
necessary due to the single threaded context where the function is run.
There are some lightweight operations inside and inc_block_group starts
with two spin locks so there's nothing we'd be losing with disabled
preemption from the caller.