linux-kernel - Re: [PATCH] sysfs: move assignment to be under lock in sysfs_remove

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Message-ID: <20131030132831.GB27632@htj.dyndns.org>
Date:	Wed, 30 Oct 2013 09:28:31 -0400
From:	Tejun Heo <tj@...nel.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sysfs: move assignment to be under lock in
 sysfs_remove_dir()

Hello,

On Tue, Oct 29, 2013 at 10:29:43PM -0700, Eric W. Biederman wrote:
> I never actually looked deeply into it, and I was working from several
> year old memory and a quick skim of the patch when I asked the question.
> 
> The protection we have previous to this patch is that syfs_remove_dir is
> only sane to call once.
> 
> Which makes the code that does:
> 	if (!dir_sd)
>         	return;
> in __sysfs_remove_dir very suspicious.   I expect we want a
> WARN_ON(!dir_sd);

It was always like that, probably in the same spirit as kfree() taking
NULL so that it can be easier, for example, in init failure paths.

> But the entire directory removal process and working on sysfs stopped
> being fun before I managed to get that cleaned up.  And unless I missed
> something go by Tejun is going to go generalize this thing before this
> bit gets cleaned up.  Sigh.

I kept the same behavior for kernfs_remove().  I don't think it's
something we explicitly want to clean up tho?  It's an acceptable
behavior.

> On an equally bizarre note.  I don't understand why we have a separate
> spinlock there.  Looks...  Sigh.  We use a different lock from
> everything as a premature optimization so that sysfs_remove_dir could be
> modified to just take a sysfs_dirent, and all of the kobject handling
> could be removed.
> 
> Sigh. It was never in my way and while I was working on the code that
> there was a good locking reason for doing that silly thing.

Umm... you got it completely wrong.  It's there to address a race
condition between removal and symlinking and has nothing to do with
optimization.

The current odd looking locking on removal side serves a purpose in
making it clear that it isn't synchronizing concurrent removal calls.
Maybe we should rename the lock to sysfs_symlink_target_lock and add
fat comments on both sides?  Or we can make it a mutex and exclude the
entire removal and symlinking, which would probably easier to follow.

Thanks.

-- 
tejun
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