linux-kernel - Re: About the try to remove cross-release feature entirely by Ingo

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Date:   Wed, 3 Jan 2018 11:38:33 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     Theodore Ts'o <tytso@....edu>,
        Matthew Wilcox <willy@...radead.org>,
        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 1/2/2018 1:00 AM, Theodore Ts'o wrote:
> On Mon, Jan 01, 2018 at 02:18:55AM -0800, Matthew Wilcox wrote:
>>> 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.
> 
> It's not just the socket lock, but any of the locks/mutexes/"waiters"
> that might be taken in the TCP code path and below, including in the
> NIC driver.
> 
>> 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.
> 
> But how do you know which of the locks taken in the networking stack
> are for the NBD versus the NFS sockets?  What manner of horrific
> abstraction violation is going to pass that information all the way
> down to all of the locks that might be taken at the socket layer and
> below?
> 
> How is this "proper clasification" supposed to happen?  It's the
> repeated handwaving which claims this is easy which is rather
> frustrating.  The simple thing is to use a unique ID which is bumped
> for each struct sock, each struct super, struct block_device, struct
> request_queue, struct bdi, etc, but that runs into lockdep scalability
> issues.

This is what I mentioned with group ID in an example for you before.
To do that, the most important thing is to prevent running into
lockdep scalability.

-- 
Thanks,
Byungchul