linux-kernel - Re: [QUESTION] about NFS sub system between Public Kernel and Red Hat Kernel.

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Message-ID: <20120831070213.6e350dd1@corrin.poochiereds.net>
Date:	Fri, 31 Aug 2012 07:02:13 -0700
From:	Jeff Layton <jlayton@...hat.com>
To:	gchen <gang.chen@...anux.com>
Cc:	linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [QUESTION] about NFS sub system between Public Kernel and Red
 Hat Kernel.

On Fri, 31 Aug 2012 13:40:16 +0800
gchen <gang.chen@...anux.com> wrote:

> Hi linux-nfs@...r.kernel.org
> 
> I have 1 question, and also 2 conclusions which need confirm.
> 
> 
> 1) Question:
> 
> Jeff Layton said in Red Hat Bugzilla (bug 848706):
> "Have configuration where the same host is acting as both NFS client
> and server. That's a configuration known to cause deadlocks."
> 
> Does it mean that the public Linux kernel (not Red Hat) also can cause
> deadlocks if NFS client and server are under the same machine ?
> 

Yes.

> 
> 2) Confirm 1: (better by Jeff Layton)
> 
> For function nfs_commit_set_lock in ./fs/nfs/write.c
> 
> for latest public kernel version:
> the parameters of out_of_line_wait_on_bit_lock() are
> (&nfsi->flags, NFS_INO_COMMIT, nfs_wait_killable, TASK_KILLABLE)
> for Red Hat kernel version: kernel-2.6.18-308.4.1.el5
> the parameters of out_of_line_wait_on_bit_lock() are
> (&nfsi->flags, NFS_INO_COMMIT,
> nfs_wait_bit_uninterruptible, TASK_UNINTERRUPTIBLE)
> 
> It means for red hat version:
> when deadlock occurs, we can not boot machine in normal way
> (it is true for my test machine, the deadlock task can not be killed)
> It means for public kernel version:
> "Assume deadlock occurs", we can still boot machine in normal way,
> because the task can be killed.
> 
> Is what I said above correct ?
> 

Not sure I understand your question. RHEL5 doesn't have support for
TASK_KILLABLE, and I didn't backport it, hence the difference in that
function.

> 
> 3) Confirm 2:
> 
> Is LTP (Linux Test Project) still a suitable test tools for public kernel ?
> (for ltp-full-20100331.gz stress test, it mounts NFS on local machine,
> and the latest LTP ltp-full-20120401.bz2 also seems the same).
> 

That I'm not sure of. All I can tell you is that mounts over loopback
(or similar configurations) are easily deadlockable under load.

-- 
Jeff Layton <jlayton@...hat.com>
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