Re: [PATCH 04/24] sysfs: Normalize removing sysfs directories.

From: Eric W. Biederman
Date: Sat May 30 2009 - 09:08:11 EST

Tejun Heo <tj@xxxxxxxxxx> 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.

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