Date: Wed, 10 May 2017 13:23:22 -0400
From: "J. Bruce Fields" <bfields@fieldses.org>
To: Paul Menzel <pmenzel@molgen.mpg.de>
Cc: Brian Foster <bfoster@redhat.com>, linux-nfs@vger.kernel.org,
        linux-xfs@vger.kernel.org, it+linux-nfs@molgen.mpg.de,
        Christoph Hellwig <hch@lst.de>,
        Jeff Layton <jlayton@poochiereds.net>
Subject: Re: Locking problems with Linux 4.9 with NFSD and `fs/iomap.c`
Message-ID: <20170510172322.GA8470@fieldses.org>
References: <de171adc29600902515d1424fd800678@molgen.mpg.de>
 <20170508131843.GB29840@bfoster.bfoster>
 <90423117-4c49-5f61-2dcf-abf6d77c7ba3@molgen.mpg.de>
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In-Reply-To: <90423117-4c49-5f61-2dcf-abf6d77c7ba3@molgen.mpg.de>
Sender: linux-nfs-owner@vger.kernel.org

On Wed, May 10, 2017 at 11:08:52AM +0200, Paul Menzel wrote:
> On 05/08/17 15:18, Brian Foster wrote:
> >>NFS doesn’t respond to a null call.
> >
> >What exactly is a NULL call?
> 
> Sorry for not making that clear for non-NFS people. From *NFS
> Version 3 Protocol Specification* [1]:
> 
> >Procedure NULL does not do any work. It is made available to
> >allow server response testing and timing.

NFSD has a fixed number of threads for processing requests, so if one of
them hangs in the filesystem while holding a lock, all of them are
likely to eventually end up waiting for that lock, and there will be
none left even to answer NULL requests.  So this is a pretty normal
symptom of a deadlock or similar problem.

--b.