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SubjectRe: [RFC PATCH] cifs: Fix possible deadlock with cifs and work queues
On Wed, 19 Mar 2014 20:34:07 +0100
Peter Zijlstra <peterz@infradead.org> wrote:

> On Wed, Mar 19, 2014 at 03:12:52PM -0400, Steven Rostedt wrote:
> > My question to Tejun is, if we create another workqueue, to add the
> > rdata->work to, would that prevent the above problem? Or what other
> > fixes can we do?
>
> The way I understand workqueues is that we cannot guarantee concurrency
> like this. It tries, but there's no guarantee.
>
> WQ_MAX_ACTIVE seems to be a hard upper limit of concurrent workers. So
> given 511 other blocked works, the described problem will always happen.
>
> Creating another workqueue doesn't actually create more threads.

But I noticed this:

Before patch:

# ps aux |grep cifs
root 3119 0.0 0.0 0 0 ? S< 14:17 0:00 [cifsiod]

After patch:

# ps aux |grep cifs
root 1109 0.0 0.0 0 0 ? S< 15:11 0:00 [cifsiod]
root 1111 0.0 0.0 0 0 ? S< 15:11 0:00 [cifsiord]

It looks to me that it does create new threads.

-- Steve


>
> There is the kthread_work stuff for if you want a guaranteed worker
> thread.



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