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 13:16:25 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Greg KH <gregkh@...uxfoundation.org>
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 03:20:11PM +0800, Greg KH wrote:

> > 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.

What I'm asking is what concurrency warranties does the whole thing
(debugfs_remove_recursive()) have to deal with.  IMO the overall
structure of the walk-and-remove the tree algorithm in there
is Not Nice(tm) and I'd like to understand if it needs to be kept
that way.  And the locking is confused in there - it either locks
too much, or not enough.

So... can debugfs_remove_recursive() rely upon the lack of attempts to create
new entries inside the subtree it's trying to kill?  If it can, the things
can be made simpler; if it can't, it's not locking enough; e.g. results of
simple_empty() on child won't be guaranteed to remain unchanged just as it
returns to caller.

What's more, the uses of simple_unlink()/simple_rmdir() there are not
imitiating the normal locking environment for ->unlink() and ->rmdir() resp. -
the victim's inode is not locked, so just for starters the call of simple_empty()
from simple_rmdir() itself is not guaranteed to give valid result.

I want to understand the overall situation.  No argument, list_empty()
in there is BS, for many reasons.  But I wonder if trying to keep the
current structure of the iterator _and_ the use of simple_rmdir()/simple_unlink()
is the right approach.