Date:   Wed, 3 Jan 2018 11:28:44 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Matthew Wilcox <willy@...radead.org>,
        Theodore Ts'o <tytso@....edu>,
        Byungchul Park <max.byungchul.park@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, david@...morbit.com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Amir Goldstein <amir73il@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        oleg@...hat.com, kernel-team@....com, daniel@...ll.ch
Subject: Re: About the try to remove cross-release feature entirely by Ingo

On 1/1/2018 7:18 PM, Matthew Wilcox wrote:
> On Sat, Dec 30, 2017 at 06:00:57PM -0500, Theodore Ts'o wrote:
>> On Sat, Dec 30, 2017 at 05:40:28PM -0500, Theodore Ts'o wrote:
>>> On Sat, Dec 30, 2017 at 12:44:17PM -0800, Matthew Wilcox wrote:
>>>>
>>>> I'm not sure I agree with this part.  What if we add a new TCP lock class
>>>> for connections which are used for filesystems/network block devices/...?
>>>> Yes, it'll be up to each user to set the lockdep classification correctly,
>>>> but that's a relatively small number of places to add annotations,
>>>> and I don't see why it wouldn't work.
>>>
>>> I was exagerrating a bit for effect, I admit.  (but only a bit).
> 
> I feel like there's been rather too much of that recently.  Can we stick
> to facts as far as possible, please?
> 
>>> It can probably be for all TCP connections that are used by kernel
>>> code (as opposed to userspace-only TCP connections).  But it would
>>> probably have to be each and every device-mapper instance, each and
>>> every block device, each and every mounted file system, each and every
>>> bdi object, etc.
>>
>> Clarification: all TCP connections that are used by kernel code would
>> need to be in their own separate lock class.  All TCP connections used
>> only by userspace could be in their own shared lock class.  You can't
>> use a one lock class for all kernel-used TCP connections, because of
>> the Network Block Device mounted on a local file system which is then
>> exported via NFS and squirted out yet another TCP connection problem.
> 
> So the false positive you're concerned about is write-comes-in-over-NFS
> (with socket lock held), NFS sends a write request to local filesystem,
> local filesystem sends write to block device, block device sends a
> packet to a socket which takes that socket lock.
> 
> I don't think we need to be as drastic as giving each socket its own lock
> class to solve this.  All NFS sockets can be in lock class A; all NBD
> sockets can be in lock class B; all user sockets can be in lock class
> C; etc.
> 
>> Also, what to do with TCP connections which are created in userspace
>> (with some authentication exchanges happening in userspace), and then
>> passed into kernel space for use in kernel space, is an interesting
>> question.
> 
> Yes!  I'd love to have a lockdep expert weigh in here.  I believe it's
> legitimate to change a lock's class after it's been used, essentially
> destroying it and reinitialising it.  If not, it should be because it's
> a reasonable design for an object to need different lock classes for
> different phases of its existance.

I also think it should be done ultimately. And I think it's very much
hard since it requires to change the dependency graph of lockdep but
anyway possible. It's up to lockdep maintainer's will though..

-- 
Thanks,
Byungchul

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