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Date:	Thu, 3 Jan 2013 17:03:09 -0500
From:	Tejun Heo <tj@...nel.org>
To:	"Adamson, Dros" <Weston.Adamson@...app.com>
Cc:	"J. Bruce Fields" <bfields@...ldses.org>,
	"Myklebust, Trond" <Trond.Myklebust@...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.

Hello, guys.

On Thu, Jan 03, 2013 at 04:28:37PM +0000, Adamson, Dros wrote:
> The deadlock we were seeing was:
> 
> - task A gets queued on rpciod workqueue and assigned kworker-0:0
> - task B gets queued on rpciod workqueue and assigned the same kworker (kworker-0:0)
> - task A gets run, calls rpc_shutdown_client(), which will loop forever waiting for task B to run rpc_async_release()
> - task B will never run rpc_async_release() - it can't run until kworker-0:0 is free, which won't happen until task A (rpc_shutdown_client) is done
> 
> The same deadlock happened when we tried queuing the tasks on a
> different workqueues -- queue_work() assigns the task to a kworker
> thread and it's luck of the draw if it's the same kworker as task A.
> We tried the different workqueue options, but nothing changed this
> behavior.

Work items don't get assigned to workers on queueing.  Idle workers
pick up work items.  A work item is directly assigned to a specific
worker iff the worker is already executing that specific work item or
the new work item is "linked" to the one it's currently executing.
Currently, the only case where a linked work item is used is when
flushing which is guaranteed to not introduce dependency the other way
around.

So, your diagnosis looks wrong to me.  If such problem existed, we
would be seeing deadlocks all over the place.

> Once a work struct is queued, there is no way to back out of the
> deadlock.  From kernel/workqueue.c:insert_wq_barrier comment:

Yes, there are.  cancel_work[_sync]() do exactly that.

>  * Currently, a queued barrier can't be canceled.  This is because
>  * try_to_grab_pending() can't determine whether the work to be
>  * grabbed is at the head of the queue and thus can't clear LINKED
>  * flag of the previous work while there must be a valid next work
>  * after a work with LINKED flag set.
> 
> So once a work struct is queued and there is an ordering dependency
> (i.e. task A is before task B), there is no way to back task B out -
> so we can't just call cancel_work() or something on task B in
> rpc_shutdown_client.

A *barrier* can't be canceled.  A barrier is used only to flush work
items.  The above comment means that we currently don't (or can't)
support canceling flush_work().  It has *nothing* to do with canceling
regular work items.  You can cancel work items fine.

> The root of our issue is that rpc_shutdown_client is never safe to
> call from a workqueue context - it loops until there are no more
> tasks, marking tasks as killed and waiting for them to be cleaned up
> in each task's own workqueue context. Any tasks that have already
> been assigned to the same kworker thread will never have a chance to
> run this cleanup stage.
>
> When fixing this deadlock, Trond and I discussed changing how
> rpc_shutdown_client works (making it workqueue safe), but Trond felt
> that it'd be better to just not call it from a workqueue context and
> print a warning if it is.
>
> IIRC we tried using different workqueues with WQ_MEM_RECLAIM (with
> no success), but I'd argue that even if that did work it would still
> be very easy to call rpc_shutdown_client from the wrong context and
> MUCH harder to detect it.  It's also unclear to me if setting rpciod
> workqueue to WQ_MEM_RECLAIM would limit it to one kworker, etc...

It looks like you guys ended up in a weird place misled by wrong
analysis.  Unless you require more than one concurrent execution on
the same workqueue, WQ_MEM_RECLAIM guarantees forward progress.  It
won't deadlock because "a different work item is queued to the same
worker".  The whole thing is designed *exactly* to avoid problems like
that.  So, I'd strongly recommend looking again at why the deadlocks
are occurring.

Thanks.

-- 
tejun
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