Re: [PATCH v11 1/4] xfs: Refactor xfs_isilocked()

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

On Mon, Oct 12, 2020 at 12:03:08PM -0400, Brian Foster wrote:
> On Fri, Oct 09, 2020 at 09:55:12PM +0200, 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>
> > Suggested-by: Darrick J. Wong <darrick.wong@oracle.com>
> > Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
> > ---
> >  fs/xfs/xfs_inode.c | 48 ++++++++++++++++++++++++++++++++++++++--------
> >  fs/xfs/xfs_inode.h | 21 +++++++++++++-------
> >  2 files changed, 54 insertions(+), 15 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index c06129cffba9..7c1ceb4df4ec 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -345,9 +345,43 @@ xfs_ilock_demote(
> >  }
> >  
> >  #if defined(DEBUG) || defined(XFS_WARN)
> > -int
> > +static inline bool
> > +__xfs_rwsem_islocked(
> > +	struct rw_semaphore	*rwsem,
> > +	int			lock_flags)
> > +{
> > +	int			arg;
> > +
> > +	if (!debug_locks)
> > +		return rwsem_is_locked(rwsem);
> > +
> > +	if (lock_flags & (1 << XFS_SHARED_LOCK_SHIFT)) {
> > +		/*
> > +		 * The caller could be asking if we have (shared | excl)
> > +		 * access to the lock. Ask lockdep if the rwsem is
> > +		 * locked either for read or write access.
> > +		 *
> > +		 * The caller could also be asking if we have only
> > +		 * shared access to the lock. Holding a rwsem
> > +		 * write-locked implies read access as well, so the
> > +		 * request to lockdep is the same for this case.
> > +		 */
> > +		arg = -1;
> > +	} else {
> > +		/*
> > +		 * The caller is asking if we have only exclusive access
> > +		 * to the lock. Ask lockdep if the rwsem is locked for
> > +		 * write access.
> > +		 */
> > +		arg = 0;
> > +	}
> 
> Are these arg values documented somewhere? A quick look at the function
> below didn't show anything..

Alas, no. :(

If you trace lockdep_is_held_type -> lock_is_held_type -> __lock_is_held
then you'll notice that "if (read == -1" bit, but none of those
functions are documented.

So I have no if that's /really/ permanent, other than to say that it
exists because Dave and Christoph and I requested it years ago and
commit f8319483f57f1 has been unchanged since 2016.

--D

> Also, I find the pattern of shifting in the caller slightly confusing,
> particularly with the 'lock_flags' name being passed down through the
> caller. Any reason we couldn't pass the shift value as a parameter and
> do the shift at the top of the function so the logic is clear and in one
> place?
> 
> > +
> > +	return lockdep_is_held_type(rwsem, arg);
> > +}
> > +
> > +bool
> >  xfs_isilocked(
> > -	xfs_inode_t		*ip,
> > +	struct xfs_inode	*ip,
> >  	uint			lock_flags)
> >  {
> >  	if (lock_flags & (XFS_ILOCK_EXCL|XFS_ILOCK_SHARED)) {
> ...
> > diff --git a/fs/xfs/xfs_inode.h b/fs/xfs/xfs_inode.h
> > index e9a8bb184d1f..77776af75c77 100644
> > --- a/fs/xfs/xfs_inode.h
> > +++ b/fs/xfs/xfs_inode.h
> > @@ -268,12 +268,19 @@ static inline void xfs_ifunlock(struct xfs_inode *ip)
> >   * Bit ranges:	1<<1  - 1<<16-1 -- iolock/ilock modes (bitfield)
> >   *		1<<16 - 1<<32-1 -- lockdep annotation (integers)
> >   */
> > -#define	XFS_IOLOCK_EXCL		(1<<0)
> > -#define	XFS_IOLOCK_SHARED	(1<<1)
> > -#define	XFS_ILOCK_EXCL		(1<<2)
> > -#define	XFS_ILOCK_SHARED	(1<<3)
> > -#define	XFS_MMAPLOCK_EXCL	(1<<4)
> > -#define	XFS_MMAPLOCK_SHARED	(1<<5)
> > +
> > +#define XFS_IOLOCK_FLAG_SHIFT	0
> > +#define XFS_ILOCK_FLAG_SHIFT	2
> > +#define XFS_MMAPLOCK_FLAG_SHIFT	4
> > +
> > +#define XFS_SHARED_LOCK_SHIFT	1
> > +
> > +#define XFS_IOLOCK_EXCL		(1 << (XFS_IOLOCK_FLAG_SHIFT))
> > +#define XFS_IOLOCK_SHARED	(XFS_IOLOCK_EXCL << (XFS_SHARED_LOCK_SHIFT))
> > +#define XFS_ILOCK_EXCL		(1 << (XFS_ILOCK_FLAG_SHIFT))
> > +#define XFS_ILOCK_SHARED	(XFS_ILOCK_EXCL << (XFS_SHARED_LOCK_SHIFT))
> > +#define XFS_MMAPLOCK_EXCL	(1 << (XFS_MMAPLOCK_FLAG_SHIFT))
> > +#define XFS_MMAPLOCK_SHARED	(XFS_MMAPLOCK_EXCL << (XFS_SHARED_LOCK_SHIFT))
> >  
> 
> Any reason for the extra params around the shift values?
> 
> Brian
> 
> >  #define XFS_LOCK_MASK		(XFS_IOLOCK_EXCL | XFS_IOLOCK_SHARED \
> >  				| XFS_ILOCK_EXCL | XFS_ILOCK_SHARED \
> > @@ -412,7 +419,7 @@ 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(struct xfs_inode *, uint);
> >  uint		xfs_ilock_data_map_shared(struct xfs_inode *);
> >  uint		xfs_ilock_attr_map_shared(struct xfs_inode *);
> >  
> > -- 
> > 2.26.2
> > 
>