Re: [PATCH v6 1/4] xfs: Refactor xfs_isilocked()

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Thu, Feb 27, 2020 at 09:36:33PM +0100, Pavel Reichl wrote:
> Refactor xfs_isilocked() to use newly introduced __xfs_rwsem_islocked().
> __xfs_rwsem_islocked() is a helper function which encapsulates checking
> state of rw_semaphores hold by inode.
> 
> Signed-off-by: Pavel Reichl <preichl@redhat.com>
> Suggested-by: Dave Chinner <dchinner@redhat.com>
> Suggested-by: Eric Sandeen <sandeen@redhat.com>
> ---
> Changes from V5:
> 	Drop shared flag from __xfs_rwsem_islocked()
> 
> 
>  fs/xfs/xfs_inode.c | 42 ++++++++++++++++++++++++++----------------
>  fs/xfs/xfs_inode.h |  2 +-
>  2 files changed, 27 insertions(+), 17 deletions(-)
> 
> diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> index c5077e6326c7..4faf7827717b 100644
> --- a/fs/xfs/xfs_inode.c
> +++ b/fs/xfs/xfs_inode.c
> @@ -345,32 +345,42 @@ xfs_ilock_demote(
>  }
>  
>  #if defined(DEBUG) || defined(XFS_WARN)
> -int
> +static inline bool
> +__xfs_rwsem_islocked(
> +	struct rw_semaphore	*rwsem,
> +	bool			excl)
> +{
> +	if (!rwsem_is_locked(rwsem))
> +		return false;

So, uh, I finally made the time to dig through what exactly lockdep
provides as far as testing functions, and came up with the following
truth table for the old xfs_isilocked behavior w.r.t. IOLOCK:

(nolockdep corresponds to debug_locks == 0)

RWSEM STATE		PARAMETERS TO XFS_ISILOCKED:
			SHARED	EXCL	SHARED | EXCL
readlocked		y	n	y
writelocked		y	y	y
unlocked		n	n	n
nolockdep readlocked	y	y	y
nolockdep writelocked	y	y	y
nolockdep unlocked	n	y	n

Note that EXCL × nolockdep_unlocked returns an incorrect result, but
because we only use it with ASSERTs there haven't been any failures
reported.

And here's your new version:

readlocked		y	y	y
writelocked		y	n	n
unlocked		n	n	n
nolockdep readlocked	y	y	y
nolockdep writelocked	y	y	y
nolockdep unlocked	n	n	n

Thanks for fixing the false positive that I mentioned above.

> +
> +	if (debug_locks && excl)
> +		return lockdep_is_held_type(rwsem, 1);

This is wrong, the second parameter of lockdep_is_held_type is 0 to test
if the rwsem is write-locked; 1 to test if it is read-locked; or -1 to
test if the rwsem is read or write-locked.

So, this function's call signature should change so that callers can
communicate both _SHARED and _EXCL; and then you can pick the correct
"r" parameter value for the lockdep_is_held_type() call.  Then all of
this becomes:

	if !debug_locks:
		return rwsem_is_locked(rwsem)

	if shared and excl:
		r = -1
	elif shared:
		r = 1
	else:
		r = 0
	return lockdep_is_held_type(rwsem, r)

Note also that you don't necessarily need to pass shared and excl as
separate parameters (as you did in v3); the XFS_*LOCK_{EXCL,SHARED}
definitions enable you to take care of all that with some clever bit
shifting and masking.

--D

> +
> +	return true;
> +}
> +
> +bool
>  xfs_isilocked(
> -	xfs_inode_t		*ip,
> +	struct xfs_inode	*ip,
>  	uint			lock_flags)
>  {
> -	if (lock_flags & (XFS_ILOCK_EXCL|XFS_ILOCK_SHARED)) {
> -		if (!(lock_flags & XFS_ILOCK_SHARED))
> -			return !!ip->i_lock.mr_writer;
> -		return rwsem_is_locked(&ip->i_lock.mr_lock);
> +	if (lock_flags & (XFS_ILOCK_EXCL | XFS_ILOCK_SHARED)) {
> +		return __xfs_rwsem_islocked(&ip->i_lock.mr_lock,
> +				(lock_flags & XFS_ILOCK_EXCL));
>  	}
>  
> -	if (lock_flags & (XFS_MMAPLOCK_EXCL|XFS_MMAPLOCK_SHARED)) {
> -		if (!(lock_flags & XFS_MMAPLOCK_SHARED))
> -			return !!ip->i_mmaplock.mr_writer;
> -		return rwsem_is_locked(&ip->i_mmaplock.mr_lock);
> +	if (lock_flags & (XFS_MMAPLOCK_EXCL | XFS_MMAPLOCK_SHARED)) {
> +		return __xfs_rwsem_islocked(&ip->i_mmaplock.mr_lock,
> +				(lock_flags & XFS_MMAPLOCK_EXCL));
>  	}
>  
> -	if (lock_flags & (XFS_IOLOCK_EXCL|XFS_IOLOCK_SHARED)) {
> -		if (!(lock_flags & XFS_IOLOCK_SHARED))
> -			return !debug_locks ||
> -				lockdep_is_held_type(&VFS_I(ip)->i_rwsem, 0);
> -		return rwsem_is_locked(&VFS_I(ip)->i_rwsem);
> +	if (lock_flags & (XFS_IOLOCK_EXCL | XFS_IOLOCK_SHARED)) {
> +		return __xfs_rwsem_islocked(&VFS_I(ip)->i_rwsem,
> +				(lock_flags & XFS_IOLOCK_EXCL));
>  	}
>  
>  	ASSERT(0);
> -	return 0;
> +	return false;
>  }
>  #endif
>  
> diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> index 492e53992fa9..3d7ce355407d 100644
> --- a/fs/xfs/xfs_inode.h
> +++ b/fs/xfs/xfs_inode.h
> @@ -416,7 +416,7 @@ void		xfs_ilock(xfs_inode_t *, uint);
>  int		xfs_ilock_nowait(xfs_inode_t *, uint);
>  void		xfs_iunlock(xfs_inode_t *, uint);
>  void		xfs_ilock_demote(xfs_inode_t *, uint);
> -int		xfs_isilocked(xfs_inode_t *, uint);
> +bool		xfs_isilocked(xfs_inode_t *, uint);
>  uint		xfs_ilock_data_map_shared(struct xfs_inode *);
>  uint		xfs_ilock_attr_map_shared(struct xfs_inode *);
>  
> -- 
> 2.24.1
>