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Date: Thu, 9 Feb 2017 08:40:16 +0000
From: Al Viro <viro@ZenIV.linux.org.uk>
To: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@yandex-team.ru>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] proc/sysctl: drop unregistered stale dentries as soon as
 possible
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On Thu, Feb 09, 2017 at 10:36:15AM +0300, Konstantin Khlebnikov wrote:

> Ok, Thank you. I've expected that this fix isn't sane,
> 
> Maybe we could minimize changes for now. For example: keep these
> stale dentries in memory but silently unhash them in ->d_compare().
> Memory processure and reclaimer will kill them later.

->d_compare() is called by the code walking the hash chains.  What's worse,
in the most common case all we have is rcu_read_lock().  Modifying the chain
in rcu reader is no-go.  Turning __d_lookup_rcu() into a writer on the
off-chance that we'll walk onto a visibly stale sysctl dentry - even more so.

If you want to deal with that, do it right, please.  Have sysctl inodes
on a list of some kind anchored in struct ctl_table_header; insert them
there in proc_sys_make_inode(), remove - in proc_evict_inode() (or
have it pass the inode to sysctl_head_put() and do the removal there).
Use sysctl_lock for serialization.

In start_unregistering(), just before the erase_header() call, check
if the list is non-empty and if it is -
	grab sysctl_lock
	last = NULL
	walk the list
		igrab(inode we are looking at)
		if succeeded
			drop sysctl_lock
			iput(last)
			last = that inode
			d_prune_aliases(last)
			retake sysctl_lock
			// inode is still not evicted, so it's still on the list
	drop sysctl_lock
	iput(last)

list would pass through struct proc_inode, and I would probably use
hlist rather than the normal one; might be more convenient to initialize
that way.  Getting from containing struct proc_inode to inode - &ei->vfs_inode.

It's not that much work; if you have time - go for it, or remind me after
-rc1...