linux-kernel - Re: [PATCH 1/3] dcache: add a new enum type for 'dentry_d_lock

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Date:   Fri, 15 Nov 2019 15:20:11 +0800
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     yu kuai <yukuai3@...wei.com>, rafael@...nel.org,
        rostedt@...dmis.org, oleg@...hat.com, mchehab+samsung@...nel.org,
        corbet@....net, tytso@....edu, jmorris@...ei.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        zhengbin13@...wei.com, yi.zhang@...wei.com,
        chenxiang66@...ilicon.com, xiexiuqi@...wei.com
Subject: Re: [PATCH 1/3] dcache: add a new enum type for 'dentry_d_lock_class'

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@...wei.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