linux-kernel - Re: [PATCH v3 2/2] writeback: Fix false warning in inode_to

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Message-Id: <20250414144741.56f7e4162c5faa9f3fb5c2a6@linux-foundation.org>
Date: Mon, 14 Apr 2025 14:47:41 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Andreas Gruenbacher <agruenba@...hat.com>
Cc: cgroups@...r.kernel.org, Jan Kara <jack@...e.cz>, Tetsuo Handa
 <penguin-kernel@...ove.sakura.ne.jp>, Rafael Aquini <aquini@...hat.com>,
 gfs2@...ts.linux.dev, linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] writeback: Fix false warning in inode_to_wb()

On Sat, 12 Apr 2025 18:39:12 +0200 Andreas Gruenbacher <agruenba@...hat.com> wrote:

> From: Jan Kara <jack@...e.cz>
> 
> inode_to_wb() is used also for filesystems that don't support cgroup
> writeback. For these filesystems inode->i_wb is stable during the
> lifetime of the inode (it points to bdi->wb) and there's no need to hold
> locks protecting the inode->i_wb dereference. Improve the warning in
> inode_to_wb() to not trigger for these filesystems.
> 
> ...
>
> --- a/include/linux/backing-dev.h
> +++ b/include/linux/backing-dev.h
> @@ -249,6 +249,7 @@ static inline struct bdi_writeback *inode_to_wb(const struct inode *inode)
>  {
>  #ifdef CONFIG_LOCKDEP
>  	WARN_ON_ONCE(debug_locks &&
> +		     (inode->i_sb->s_iflags & SB_I_CGROUPWB) &&
>  		     (!lockdep_is_held(&inode->i_lock) &&
>  		      !lockdep_is_held(&inode->i_mapping->i_pages.xa_lock) &&
>  		      !lockdep_is_held(&inode->i_wb->list_lock)));

Is this a does-nothing now GFS2 has been altered?

Otherwise, a bogus WARN is something we'll want to eliminate from
-stable kernels also.  Are we able to identify a Fixes: for this?

Thanks.