Subject: Re: Overview of concurrency managed workqueue
From: Johannes Berg <johannes@sipsolutions.net>
To: Tejun Heo <tj@kernel.org>
Cc: mingo@elte.hu, awalls@radix.net, linux-kernel@vger.kernel.org,
       jeff@garzik.org, akpm@linux-foundation.org, rusty@rustcorp.com.au,
       cl@linux-foundation.org, dhowells@redhat.com, arjan@linux.intel.com,
       oleg@redhat.com, axboe@kernel.dk
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	 <4C18D420.8090008@kernel.org>
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Date: Wed, 16 Jun 2010 15:42:14 +0200
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On Wed, 2010-06-16 at 15:39 +0200, Tejun Heo wrote:
> On 06/16/2010 03:37 PM, Johannes Berg wrote:
> >> As multiple execution contexts are available for each wq, deadlocks
> >> around execution contexts is much harder to create.  The default
> >> workqueue, system_wq, has maximum concurrency level of 256 and unless
> >> there is a use case which can result in a dependency loop involving
> >> more than 254 workers, it won't deadlock.
> > 
> > I see a lot of stuff about the current limitations etc., but nothing
> > about code that actually _relies_ on the synchronisation properties of
> > the current wqs. We talked about that a long time ago, is it still
> > guaranteed that a single-threaded wq will serialise all work put onto
> > it? It needs to be, but I don't see you explicitly mentioning it.
> 
> Oh yeah, if you have WQ_SINGLE_CPU + max inflight of 1, works on the
> wq are fully ordered.

Ok, great, thanks. FWIW, that's pretty much all I care about right
now :)

johannes

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