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

[yukuai (C)]

On 2019/11/15 15:20, Greg KH wrote:
> 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@xxxxxxxxxx>
> > > > ---
> > > >   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@xxxxxxxxxx/
>
> 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
>
> .
The main purpose of this patchset is to fix the infinite loop in
debugfs_remove_recursive. Steven point out that simple replace
list_empty with simple_empty will cause a splat with lockdep enabled.
We try to fix it with the first two patch, do you think it's appropriate?

Thanks,
Yu Kuai