linux-kernel - Re: [NFS] 2.6.23-rc1-mm2

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Date:	Tue, 07 Aug 2007 19:08:27 -0400
From:	Trond Myklebust <trond.myklebust@....uio.no>
To:	Oleg Nesterov <oleg@...sign.ru>
Cc:	Neil Brown <neilb@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Dietrich <Marc.Dietrich@...physik.uni-giessen.de>,
	Johannes Berg <johannes@...solutions.net>,
	nfs@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: [NFS] 2.6.23-rc1-mm2

On Wed, 2007-08-08 at 02:20 +0400, Oleg Nesterov wrote:

> But. nfs4_renew_state() checks list_empty(&clp->cl_superblocks) under
> clp->cl_sem? So, if it is possible that clp->cl_renewd was scheduled
> at the time when nfs4_kill_renewd(), we can deadlock, no? Because
> nfs4_renew_state() needs clp->cl_sem to complete, but nfs4_kill_renewd()
> holds this sem, and waits for nfs4_renew_state() completion.

They both take read locks, which means that they can take them
simultaneously. AFAICS, the deadlock can only occur if something manages
to insert a request for a write lock after nfs4_kill_renewd() takes its
read lock, but before nfs4_renew_state() takes its read lock:

1) nfs4_kill_renewd()		2) nfs4_renew_state()		3) somebody else
-------------------             ------------------		-------------
read lock
wait on (2) to complete
								write lock <waits on (1)>
				read lock <waits on (3),
					because rw_semaphores
					don't allow a read lock
					request to jump a write
					lock request>

however as I explained earlier, the only process that can take a write
lock is the reclaimer daemon, but we _know_ that cannot be running (for
one thing, the reference count on nfs_client is zero, for the other,
there are no superblocks).

Cheers
  Trond

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