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Message-ID: <m1d49qoi1o.fsf@fess.ebiederm.org>
Date: Sat, 30 May 2009 06:07:47 -0700
From: ebiederm@...ssion.com (Eric W. Biederman)
To: Tejun Heo <tj@...nel.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
Greg Kroah-Hartman <gregkh@...e.de>,
linux-kernel@...r.kernel.org,
Cornelia Huck <cornelia.huck@...ibm.com>,
linux-fsdevel@...r.kernel.org, Kay Sievers <kay.sievers@...y.org>,
Greg KH <greg@...ah.com>,
"Eric W. Biederman" <ebiederm@...stanetworks.com>
Subject: Re: [PATCH 04/24] sysfs: Normalize removing sysfs directories.
Tejun Heo <tj@...nel.org> writes:
> Eric W. Biederman wrote:
>> Also, I'm quite uncomfortable with these things
>>> being done in non-atomic manner. It can be made to work but things
>>> like this can lead to subtle race conditions and with the kind of
>>> layering we put on top of sysfs (kobject, driver model, driver
>>> midlayers and so on), it isn't all that easy to verify what's going
>>> on, so NACK for this one.
>>
>> Total nonsense.
>>
>> Mucking about with sysfs after we start deleting a directory is a bug.
>> At worst my change makes a buggy race slightly less deterministic.
>>
>> I am not ready to consider keeping the current unnecessary atomic
>> removal step. That unnecessary atomicity makes the following patches
>> more difficult, and requires a lot of unnecessary retesting.
>>
>> What do you think the extra unnecessary atomicity helps protect?
>
> It's just not a clean API. When people are trying to code things way
> up in the stack, they aren't likely to look up the code to see what
> assumptions are being made especially when the stack is deep and
> complex and sysfs is near the bottom of the tall stack. IMHO
> implementing the usually expected semantics at this depth is worth
> every effort. It's just good implementation style which might look
> like wasted effort but will harden the stack in the long run. Plus,
> it's not like making it atomic is difficult or anything.
I guess we are going to have to disagree on this one.
My take is simply that a correct user has to wait until no one else
can find the kobject before calling kobject_del. At which point
races are impossible, and it doesn't matter if sysfs_mutex is held
across the entire operation.
For the long term I still intend to kill __sysfs_remove_dir. Just
not in this patch series.
Eric
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