linux-kernel - Re: nfsd oops on Linus' current tree.

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Date:	Fri, 21 Dec 2012 18:08:49 -0500
From:	"J. Bruce Fields" <bfields@...ldses.org>
To:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
Cc:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"Adamson, Dros" <Weston.Adamson@...app.com>
Subject: Re: nfsd oops on Linus' current tree.

On Fri, Dec 21, 2012 at 06:40:54PM +0000, Myklebust, Trond wrote:
> On Fri, 2012-12-21 at 13:08 -0500, J. Bruce Fields wrote:
> > On Fri, Dec 21, 2012 at 10:33:48AM -0500, Dave Jones wrote:
> > > Did a mount from a client (also running Linus current), and the
> > > server spat this out..
> > > 
> > > [ 6936.306135] ------------[ cut here ]------------
> > > [ 6936.306154] WARNING: at net/sunrpc/clnt.c:617 rpc_shutdown_client+0x12a/0x1b0 [sunrpc]()
> > 
> > This is a warning added by 168e4b39d1afb79a7e3ea6c3bb246b4c82c6bdb9
> > "SUNRPC: add WARN_ON_ONCE for potential deadlock", pointing out that
> > nfsd is calling shutdown_client from a workqueue, which is a problem
> > because shutdown_client has to wait on rpc tasks that run on a
> > workqueue.
> > 
> > I don't believe there's any circular dependency among the workqueues
> > (we're calling shutdown_client from callback_wq, not rpciod_workqueue),
> 
> We were getting deadlocks with rpciod when calling rpc_shutdown_client
> from the nfsiod workqueue.
> 
> The problem here is that the workqueues all run using the same pool of
> threads, and so you can get "interesting" deadlocks when one of these
> threads has to wait for another one.

OK, coming back after reviewing Documentation/workqueue.txt: the
workqueues potentially involved here are rpciod and the server's
laundromat and callback workqueues.

The former is created with

	alloc_workqueue("rpciod", WQ_MEM_RECLAIM, 1);

and the latter two are both created with
create_singlethread_workqueue(), which also sets WQ_MEM_RECLAIM.

As I understand it, that ensures that each of the three has at least one
"rescuer" thread dedicated to it--so there shouldn't be any deadlock as
long as there's no circular dependency among the three.

So think this is a false positive--am I missing something?

> > but 168e4b39d1afb.. says that we could get a deadlock if both are
> > running on the same kworker thread.
> > 
> > I'm not sure what to do about that.
> > 
> 
> The question is if you really do need the call to rpc_killall_tasks and
> the synchronous wait for completion of old tasks? If you don't care,
> then we could just have you call rpc_release_client() in order to
> release your reference on the rpc_client.

No, the waiting's intentional: for example, the laundromat wq is what's
responsible for reaping expired clients, and it wants to wait for
callback rpc's to exit.  (If I remember right they may reference data
structures we're about to clean up).

--b.
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