Re: [PATCH 1/3] dcache: add a new enum type for 'dentry_d_lock_class'

Greg KH
2019-11-15T07:20:19Z

On Fri, Nov 15, 2019 at 04:12:43AM +0000, Al Viro wrote:
> On Fri, Nov 15, 2019 at 11:27:59AM +0800, Greg KH wrote:
> > On Fri, Nov 15, 2019 at 11:27:50AM +0800, yu kuai wrote:
> > > 'dentry_d_lock_class' can be used for spin_lock_nested in case lockdep
> > > confused about two different dentry take the 'd_lock'.
> > >
> > > However, a single 'DENTRY_D_LOCK_NESTED' may not be enough if more than
> > > two dentry are involed. So, and in 'DENTRY_D_LOCK_NESTED_2'
> > >
> > > Signed-off-by: yu kuai <yukuai3@huawei.com>
> > > ---
> > > include/linux/dcache.h | 3 ++-
> > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/dcache.h b/include/linux/dcache.h
> > > index 10090f1..8eb84ef 100644
> > > --- a/include/linux/dcache.h
> > > +++ b/include/linux/dcache.h
> > > @@ -129,7 +129,8 @@ struct dentry {
> > > enum dentry_d_lock_class
> > > {
> > > DENTRY_D_LOCK_NORMAL, /* implicitly used by plain spin_lock() APIs. */
> > > - DENTRY_D_LOCK_NESTED
> > > + DENTRY_D_LOCK_NESTED,
> > > + DENTRY_D_LOCK_NESTED_2
> >
> > You should document this, as "_2" does not make much sense to anyone
> > only looking at the code :(
> >
> > Or rename it better.
>
> FWIW, I'm not sure it's a good solution. What are the rules for callers
> of that thing, anyway? If it can be called when somebody is creating
> more files in that subtree, we almost certainly will have massive
> problems with the lifetimes of underlying objects...
>
> Could somebody familiar with debugfs explain how is that thing actually
> used and what is required from/promised to its callers? I can try and
> grep through the tree and guess what the rules are, but I've way too
> much on my platter right now and I don't want to get sidetracked into yet
> another tree-wide search and analysis session ;-/

Yu wants to use simple_empty() in debugfs_remove_recursive() instead of
manually checking:
if (!list_empty(&child->d_subdirs)) {

See patch 3 of this series for that change and why they feel it is
needed:
https://lore.kernel.org/lkml/1573788472-87426-4-git-send-email-yukuai3@huawei.com/

As to if patch 3 really is needed, I'll leave that up to Yu given that I
thought we had resolved these types of issues already a year or so ago.

thanks,

greg k-h