Re: sysfs: tagged directories not merged completely yet

[ebiederm@xmission.com (Eric W. Biederman)]

Tejun Heo <tj@kernel.org> writes:

> Hmmm... I'm probably missing something (and being lazy) but how does it
> guarantee the validity of the next pointer after dropping the rcu lock?

So in next_tid which is essentially what we would be doing.
I grab the rcu_lock.  Check something to see if the task I have
is still on the list, if it is then I know the next is valid until
the end of the rcu grace period.  Then I follow the next pointer,
and grab the lock again.

rcu is pain to get right but at least it is localized pain.

>> I'm still not certain how we can get the lock ordering so it doesn't
>> cause us problems.  I will look at revalidation and what the other
>> distributed filesystems are doing and see if that might work.  If it
>> doesn't we need refactor the VFS locking.
>
> Yeah, if we can make sysfs behave like other distributed filesystems, it
> would be great.  :-)

I don't think we can make it work but I think we need to exhaust that
avenue before saying that the VFS has to change.

Eric