Re: [PATCH 1/4] xfs: change xfs_isilocked() to always use lockdep()

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

On Thu, Jan 30, 2020 at 09:18:19AM +1100, Dave Chinner wrote:
> On Tue, Jan 28, 2020 at 03:55:25PM +0100, Pavel Reichl wrote:
> > mr_writer is obsolete and the information it contains is accesible
> > from mr_lock.
> > 
> > Signed-off-by: Pavel Reichl <preichl@redhat.com>
> > ---
> >  fs/xfs/xfs_inode.c | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/xfs/xfs_inode.c b/fs/xfs/xfs_inode.c
> > index c5077e6326c7..32fac6152dc3 100644
> > --- a/fs/xfs/xfs_inode.c
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -352,13 +352,17 @@ xfs_isilocked(
> >  {
> >  	if (lock_flags & (XFS_ILOCK_EXCL|XFS_ILOCK_SHARED)) {
> >  		if (!(lock_flags & XFS_ILOCK_SHARED))
> > -			return !!ip->i_lock.mr_writer;
> > +			return !debug_locks ||
> > +				lockdep_is_held_type(&ip->i_lock.mr_lock, 0);
> >  		return rwsem_is_locked(&ip->i_lock.mr_lock);
> >  	}
> >  
> >  	if (lock_flags & (XFS_MMAPLOCK_EXCL|XFS_MMAPLOCK_SHARED)) {
> >  		if (!(lock_flags & XFS_MMAPLOCK_SHARED))
> > -			return !!ip->i_mmaplock.mr_writer;
> > +			return !debug_locks ||
> > +				lockdep_is_held_type(
> > +					&ip->i_mmaplock.mr_lock,
> > +					0);
> >  		return rwsem_is_locked(&ip->i_mmaplock.mr_lock);
> >  	}
> 
> Ok, so this code is only called from ASSERT() statements, which
> means this turns off write lock checking for XFS debug kernels if
> lockdep is not enabled. Hence I think these checks need to be
> restructured to be based around rwsem_is_locked() first and lockdep
> second.
> 
> That is:
> 
> /* In all implementations count != 0 means locked */
> static inline int rwsem_is_locked(struct rw_semaphore *sem)
> {
>         return atomic_long_read(&sem->count) != 0;
> }
> 
> This captures both read and write locks on the rwsem, and doesn't
> discriminate at all. Now we don't have explicit writer lock checking
> in CONFIG_XFS_DEBUG=y kernels, I think we need to at least check
> that the rwsem is locked in all cases to catch cases where we are
> calling a function without the lock held. That will ctach most
> programming mistakes, and then lockdep will provide the
> read-vs-write discrimination to catch the "hold the wrong lock type"
> mistakes.
> 
> Hence I think this code should end up looking like this:
> 
> 	if (lock_flags & (XFS_ILOCK_EXCL|XFS_ILOCK_SHARED)) {
> 		bool locked = false;
> 
> 		if (!rwsem_is_locked(&ip->i_lock))
> 			return false;
> 		if (!debug_locks)
> 			return true;
> 		if (lock_flags & XFS_ILOCK_EXCL)
> 			locked = lockdep_is_held_type(&ip->i_lock, 0);
> 		if (lock_flags & XFS_ILOCK_SHARED)
> 			locked |= lockdep_is_held_type(&ip->i_lock, 1);
> 		return locked;
> 	}
> 
> Thoughts?

I like that a lot better, though perhaps the if body should be factored
into a separate static inline so we don't repeat that 3x.

--D

> -Dave.
> -- 
> Dave Chinner
> david@fromorbit.com