linux-kernel - Re: [PATCH v1 0/4] Fixup NLM and kNFSD file lock callbacks

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Message-ID: <20240912-zuspielen-selektiert-01ef380e0c1b@brauner>
Date: Thu, 12 Sep 2024 12:07:31 +0200
From: Christian Brauner <brauner@...nel.org>
To: Benjamin Coddington <bcodding@...hat.com>
Cc: Chuck Lever <chuck.lever@...cle.com>, Jeff Layton <jlayton@...nel.org>, 
	Amir Goldstein <amir73il@...il.com>, Neil Brown <neilb@...e.de>, Trond Myklebust <trondmy@...nel.org>, 
	Anna Schumaker <anna@...nel.org>, Jonathan Corbet <corbet@....net>, 
	Andreas Gruenbacher <agruenba@...hat.com>, Mark Fasheh <mark@...heh.com>, Joel Becker <jlbec@...lplan.org>, 
	Joseph Qi <joseph.qi@...ux.alibaba.com>, Alexander Viro <viro@...iv.linux.org.uk>, 
	Jan Kara <jack@...e.cz>, Alexander Ahring Oder Aring <aahringo@...hat.com>, 
	linux-fsdevel@...r.kernel.org, linux-nfs@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, gfs2@...ts.linux.dev, ocfs2-devel@...ts.linux.dev
Subject: Re: [PATCH v1 0/4] Fixup NLM and kNFSD file lock callbacks

On Wed, Sep 11, 2024 at 03:42:56PM GMT, Benjamin Coddington wrote:
> Last year both GFS2 and OCFS2 had some work done to make their locking more
> robust when exported over NFS.  Unfortunately, part of that work caused both
> NLM (for NFS v3 exports) and kNFSD (for NFSv4.1+ exports) to no longer send
> lock notifications to clients.
> 
> This in itself is not a huge problem because most NFS clients will still
> poll the server in order to acquire a conflicted lock, but now that I've
> noticed it I can't help but try to fix it because there are big advantages
> for setups that might depend on timely lock notifications, and we've
> supported that as a feature for a long time.
> 
> Its important for NLM and kNFSD that they do not block their kernel threads
> inside filesystem's file_lock implementations because that can produce
> deadlocks.  We used to make sure of this by only trusting that
> posix_lock_file() can correctly handle blocking lock calls asynchronously,
> so the lock managers would only setup their file_lock requests for async
> callbacks if the filesystem did not define its own lock() file operation.
> 
> However, when GFS2 and OCFS2 grew the capability to correctly
> handle blocking lock requests asynchronously, they started signalling this
> behavior with EXPORT_OP_ASYNC_LOCK, and the check for also trusting
> posix_lock_file() was inadvertently dropped, so now most filesystems no
> longer produce lock notifications when exported over NFS.
> 
> I tried to fix this by simply including the old check for lock(), but the
> resulting include mess and layering violations was more than I could accept.
> There's a much cleaner way presented here using an fop_flag, which while
> potentially flag-greedy, greatly simplifies the problem and grooms the

It's fine. I've explicitly added the fop_flags so that stuff like this
we would not want to put into f->f_mode can live there.

> way for future uses by both filesystems and lock managers alike.