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From:   Chuck Lever III <chuck.lever@oracle.com>
To:     Charles Hedrick <hedrick@rutgers.edu>
CC:     Linux NFS Mailing List <linux-nfs@vger.kernel.org>,
        "trondmy@hammerspace.com" <trondmy@hammerspace.com>,
        Bruce Fields <bfields@fieldses.org>,
        Jeff Layton <jlayton@redhat.com>, Dai Ngo <dai.ngo@oracle.com>
Subject: Re: [PATCH RFC 0/2] Fix "sleep while locked" in RELEASE_LOCKOWNER
Thread-Topic: [PATCH RFC 0/2] Fix "sleep while locked" in RELEASE_LOCKOWNER
Thread-Index: AQHYZYF+q+AyM5Ko9UyH8L7ZZq5Gca0xn2sAgABJIoA=
Date:   Thu, 26 May 2022 23:39:04 +0000
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> On May 26, 2022, at 3:17 PM, Charles Hedrick <hedrick@rutgers.edu> wrote:
>=20
> We are still stuck on NFS 3 because NFS 4 lock operations hang. Typically=
 with thunderbird, firefox, etc. I had hoped that Ubuntu 22 would fix this,=
 given the patch=20
>=20
> UNRPC: Don't call connect() more than once on a TCP socket
>=20
> If this is part of the problem, that would mean we couldn't use NFS 4 unt=
il Ubuntu 24, i.e. summer of 2025, given delays in release and deployment.
>=20
> Unfortunately I can't reproduce our problem. It doesn't show up until we'=
re halfway into our semester and loads start getting heavier.
>=20
> You say this is a long-standing issue. So are problems with NFS 4 locking=
 (and also NFS 4 delegation). If you have a patch for both of these issues =
that we could put into 5.4.0, I might be willing to test it, assuming the p=
atches are safe. We probably wouldn't know it has really fixed things for a=
t least 6 months.

Charles, this mailing list is an upstream Linux forum. There honestly isn't=
 anything we can do about Ubuntu backporting policies, and we can't help mu=
ch at all with Linux kernels as old as v5.4 unless there are known fixes in=
 later kernels. It's up to you to find those fixes, test them, and then con=
vince the stable kernel folks and your distribution to include the fix in t=
heir kernel. The folks on linux-nfs@ are little more than process observers=
 in those communities.

The RELEASE_LOCKOWNER lock inversion issue has been around forever, but it =
was exposed recently by a performance regression fix in v5.18-rc3. After th=
at point, a client can leverage the existing lock inversion bug to provoke =
a deadlock on the server using normal NFSv4 operations. That makes the RELE=
ASE_LOCKOWNER issue a potential denial-of-service in the latest kernels, wh=
ich is priority one in my book.

I stand by my statement to Linus in this morning's pull request: I currentl=
y know of no other high priority bugs in v5.18's NFS server (I'm not talkin=
g about the NFS client) under active investigation except for the one I men=
tioned in the PR. If you know of /specific/ reports of significant incorrec=
t behavior in the latest upstream Linux NFS client or server, please post l=
inks to them here, or better yet, file bugs and help the assignees to troub=
leshoot the problems.


--
Chuck Lever