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

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Date:	Thu, 3 Jan 2013 23:26:56 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Tejun Heo <tj@...nel.org>
CC:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Adamson, Dros" <Weston.Adamson@...app.com>,
	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>
Subject: Re: nfsd oops on Linus' current tree.

On Thu, 2013-01-03 at 18:11 -0500, Trond Myklebust wrote:
> On Thu, 2013-01-03 at 17:26 -0500, Tejun Heo wrote:
> > Ooh, BTW, there was a bug where workqueue code created a false
> > dependency between two work items.  Workqueue currently considers two
> > work items to be the same if they're on the same address and won't
> > execute them concurrently - ie. it makes a work item which is queued
> > again while being executed wait for the previous execution to
> > complete.
> > 
> > If a work function frees the work item, and then waits for an event
> > which should be performed by another work item and *that* work item
> > recycles the freed work item, it can create a false dependency loop.
> > There really is no reliable way to detect this short of verifying
> > every memory free.  A patch is queued to make such occurrences less
> > likely (work functions should also match for two work items considered
> > the same), but if you're seeing this, the best thing to do is freeing
> > the work item at the end of the work function.
> 
> That's interesting... I wonder if we may have been hitting that issue.
> 
> From what I can see, we do actually free the write RPC task (and hence
> the work_struct) before we call the asynchronous unlink completion...
> 
> Dros, can you see if reverting commit
> 324d003b0cd82151adbaecefef57b73f7959a469 + commit 
> 168e4b39d1afb79a7e3ea6c3bb246b4c82c6bdb9 and then applying the attached
> patch also fixes the hang on a pristine 3.7.x kernel?

Actually, we probably also need to look at rpc_free_task, so the
following patch, instead...

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com

View attachment "gnurr2.dif" of type "text/x-patch" (2061 bytes)