Re: [PATCH 1/4] kernfs: switch kernfs to use an rwsem

[Ian Kent <raven@themaw.net>]

On Sun, 2020-06-07 at 16:40 +0800, Ian Kent wrote:
> Hi Greg,
> 
> On Mon, 2020-05-25 at 13:47 +0800, Ian Kent wrote:
> > @@ -189,9 +189,9 @@ int kernfs_iop_getattr(const struct path *path,
> > struct kstat *stat,
> >  	struct inode *inode = d_inode(path->dentry);
> >  	struct kernfs_node *kn = inode->i_private;
> >  
> > -	mutex_lock(&kernfs_mutex);
> > +	down_read(&kernfs_rwsem);
> >  	kernfs_refresh_inode(kn, inode);
> > -	mutex_unlock(&kernfs_mutex);
> > +	up_read(&kernfs_rwsem);
> >  
> >  	generic_fillattr(inode, stat);
> >  	return 0;
> > @@ -281,9 +281,9 @@ int kernfs_iop_permission(struct inode *inode,
> > int mask)
> >  
> >  	kn = inode->i_private;
> >  
> > -	mutex_lock(&kernfs_mutex);
> > +	down_read(&kernfs_rwsem);
> >  	kernfs_refresh_inode(kn, inode);
> > -	mutex_unlock(&kernfs_mutex);
> > +	up_read(&kernfs_rwsem);
> >  
> >  	return generic_permission(inode, mask);
> >  }
> 
> I changed these from a write lock to a read lock late in the
> development.
> 
> But kernfs_refresh_inode() modifies the inode so I think I should
> have taken the inode lock as well as taking the read lock.
> 
> I'll look again but a second opinion (anyone) would be welcome.

I had a look at this today and came up with a couple of patches
to fix it, I don't particularly like to have to do what I did
but I don't think there's any other choice. That's because the
rb tree locking is under significant contention and changing
this back to use the write lock will adversely affect that.

But unless I can find out more about the anomalous kernel test
robot result I can't do anything!

Providing a job.yaml to reproduce it with the hardware specification
of the lkp machine it was run on and no guidelines on what that test
does and what the test needs so it can actually be reproduced isn't
that useful.

Ian