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

[Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>]

On Fri, Mar 06, 2020 at 03:16:53PM +0800, Qu Wenruo wrote:
> 
> 
> On 2020/3/6 下午2:52, madhuparnabhowmik10@gmail.com wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > 
> > 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@roeck-us.net>
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > ---
> >  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?

Thank you,
Madhuparna

> Thanks,
> Qu
> >  		btrfs_release_path(path);
> >  	}
> >  
> > +	rcu_read_lock();
> >  	list_for_each_entry_rcu(space_info, &info->space_info, list) {
> >  		if (!(btrfs_get_alloc_profile(info, space_info->flags) &
> >  		      (BTRFS_BLOCK_GROUP_RAID10 |
> > @@ -2007,7 +2008,8 @@ int btrfs_read_block_groups(struct btrfs_fs_info *info)
> >  				list)
> >  			inc_block_group_ro(cache, 1);
> >  	}
> > -
> > +	rcu_read_unlock();
> > +		
> >  	btrfs_init_global_block_rsv(info);
> >  	ret = check_chunk_block_group_mappings(info);
> >  error:
> > 
>