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SubjectRe: [PATCH 04/24] sysfs: Normalize removing sysfs directories.
James Bottomley <> writes:

> On Sat, 2009-05-30 at 06:07 -0700, Eric W. Biederman wrote:
>> Tejun Heo <> 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.
> I'm afraid this one isn't a valid assumption. If you look in SCSI,
> you'll see we do get objects after they've been removed from visibility.
> We use it as part of the state model for how our objects work (objects
> removed from visibility are dying, but we still need them to be findable
> (and gettable).

I was not precise enough. It appears I overlooked the fact that
kobject_del is not always called from kobject_put by way of

Strictly the requirement is that after kobject_del we don't add,
remove or otherwise manipulate sysfs attributes. That is we don't
call any of:


Those all either oops or BUG today if you try it. So I can't see how
a subsystem could depend on those working.

Also there is sysfs_remove_dir (on a subdirectory) aka kobject_del on
a child object after kobject_del on the parent object.

As best I can tell that only works by fluke today.

> Now, this could be altered as part of an object lifetime rewrite of SCSI
> (and I suspect a few other subsystems) but it's certainly an open
> question of whether the pain is worth the gain.

I won't tell you that sysfs, the kobject layer, or the device layer
are the best thing since sliced bread. I'm just trying to simplify
the code and get the bugs out.


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