Re: [RFC PATCH] cifs: Fix possible deadlock with cifs and work queues

[Steven Rostedt]

On Wed, 19 Mar 2014 20:34:07 +0100
Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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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