linux-kernel - Re: [PATCH V4 02/13] fs/xfs: Clarify lockdep dependency for xfs

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Date:   Fri, 21 Feb 2020 12:34:51 +1100
From:   Dave Chinner <david@...morbit.com>
To:     ira.weiny@...el.com
Cc:     linux-kernel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Christoph Hellwig <hch@....de>,
        "Theodore Y. Ts'o" <tytso@....edu>, Jan Kara <jack@...e.cz>,
        linux-ext4@...r.kernel.org, linux-xfs@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH V4 02/13] fs/xfs: Clarify lockdep dependency for
 xfs_isilocked()

On Thu, Feb 20, 2020 at 04:41:23PM -0800, ira.weiny@...el.com wrote:
> From: Ira Weiny <ira.weiny@...el.com>
> 
> xfs_isilocked() can't work fully without CONFIG_LOCKDEP.  However,
> making xfs_isilocked() dependant on CONFIG_LOCKDEP is not feasible
> because it is used for more than the i_rwsem.  Therefore a short-circuit
> was provided via debug_locks.  However, this caused confusion while
> working through the xfs locking.
> 
> Rather than use debug_locks as a flag specify this clearly using
> IS_ENABLED(CONFIG_LOCKDEP).
> 
> Signed-off-by: Ira Weiny <ira.weiny@...el.com>
> 
> ---
> Changes from V3:
> 	Reordered to be a "pre-cleanup" patch
> 
> Changes from V2:
> 	This patch is new for V3
> ---
>  fs/xfs/xfs_inode.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index c5077e6326c7..35df324875db 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -364,7 +364,7 @@ xfs_isilocked(
>  
>  	if (lock_flags & (XFS_IOLOCK_EXCL|XFS_IOLOCK_SHARED)) {
>  		if (!(lock_flags & XFS_IOLOCK_SHARED))
> -			return !debug_locks ||
> +			return !IS_ENABLED(CONFIG_LOCKDEP) ||
>  				lockdep_is_held_type(&VFS_I(ip)->i_rwsem, 0);

This breaks expected lockdep behaviour.

We need to use debug_locks here because lockdep turns off lock
checking via debug_locks when lockdep encounters a locking
inconsistency.  We only want to know about the first locking
problem, not spew cascading lock problems over and over once we
already know there is a locking problem.

IOWs, checking debug_locks is required here for the same reason it
is used in lockdep_assert_held_{read/write}(). essentially we are
open coding lockdep_assert_held_write() here because this function
is only called from within ASSERT() statements and we don't want
multiple WARN/BUGs being issued when this triggers....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com