linux-kernel - Re: [RFC PATCH] vfs: add inode lockdep assertions

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Date:   Wed, 6 Sep 2023 19:33:48 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     Bernd Schubert <bernd.schubert@...tmail.fm>,
        Mateusz Guzik <mjguzik@...il.com>, brauner@...nel.org,
        viro@...iv.linux.org.uk, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: [RFC PATCH] vfs: add inode lockdep assertions

On Wed, Sep 06, 2023 at 10:07:24AM -0700, Darrick J. Wong wrote:
> On Wed, Sep 06, 2023 at 05:00:14PM +0100, Matthew Wilcox wrote:
> > +++ b/fs/xfs/xfs_inode.c
> > @@ -361,7 +361,7 @@ xfs_isilocked(
> >  {
> >  	if (lock_flags & (XFS_ILOCK_EXCL|XFS_ILOCK_SHARED)) {
> >  		if (!(lock_flags & XFS_ILOCK_SHARED))
> > -			return !!ip->i_lock.mr_writer;
> > +			return rwsem_is_write_locked(&ip->i_lock.mr_lock);
> 
> You'd be better off converting this to:
> 
> 	return __xfs_rwsem_islocked(&ip->i_lock.mr_lock,
> 				(lock_flags & XFS_ILOCK_SHARED));
> 
> And then fixing __xfs_rwsem_islocked to do:
> 
> static inline bool
> __xfs_rwsem_islocked(
> 	struct rw_semaphore	*rwsem,
> 	bool			shared)
> {
> 	if (!debug_locks) {
> 		if (!shared)
> 			return rwsem_is_write_locked(rwsem);
> 		return rwsem_is_locked(rwsem);
> 	}
> 
> 	...
> }

Thanks.

> > +++ b/include/linux/rwsem.h
> > @@ -72,6 +72,11 @@ static inline int rwsem_is_locked(struct rw_semaphore *sem)
> >  	return atomic_long_read(&sem->count) != 0;
> >  }
> >  
> > +static inline int rwsem_is_write_locked(struct rw_semaphore *sem)
> > +{
> > +	return atomic_long_read(&sem->count) & 1;
> 
> 
> atomic_long_read(&sem->count) & RWSEM_WRITER_LOCKED ?

Then this would either have to be in rwsem.c or we'd have to move the
definition of RWSEM_WRITER_LOCKED to rwsem.h.  All three options are
kind of bad.  I think I hate the bare '1' least.