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Message-ID: <1261150788.20899.299.camel@laptop>
Date: Fri, 18 Dec 2009 16:39:48 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Tejun Heo <tj@...nel.org>, awalls@...ix.net,
linux-kernel@...r.kernel.org, jeff@...zik.org, mingo@...e.hu,
akpm@...ux-foundation.org, jens.axboe@...cle.com,
rusty@...tcorp.com.au, cl@...ux-foundation.org,
dhowells@...hat.com, arjan@...ux.intel.com, avi@...hat.com,
johannes@...solutions.net, andi@...stfloor.org
Subject: Re: workqueue thing
On Fri, 2009-12-18 at 07:30 -0800, Linus Torvalds wrote:
>
> On Fri, 18 Dec 2009, Peter Zijlstra wrote:
>
> > > r1. The first design goal of cmwq is solving the issues the current
> > > workqueue implementation has including hard to detect
> > > deadlocks,
> >
> > lockdep is quite proficient at finding these these days.
>
> I don't think so.
>
> The reason it is not is that workqueues fundamentally do _different_
> things in the same context, adn lockdep has no clue what-so-ever.
>
> IOW, if you hold a lock, and then do 'flush_workqueue()', lockdep has no
> idea that maybe one of the entries on a workqueue might need the lock that
> you are holding. But I don't think lockdep sees the dependency that gets
> created by the flush - because it's not a direct code execution
> dependency.
>
> It's not a deadlock _directly_ due to lock ordering, but indirectly due to
> waiting for unrelated code that needs locks.
>
> Now, maybe lockdep could be _taught_ to consider workqueues themselves to
> be 'locks', and ordering those pseudo-locks wrt the real locks they take.
> So if workqueue Q takes lock A, the fact that it is _taken_ in a workqueue
> makes the ordering be Q->A. Then, if somebody does a "flush_workqueue"
> while holding lock B, the flush implies a "lock ordering" of B->Q (where
> "Q" is the set of all workqueues that could be flushed).
That's exactly what it does..
4e6045f134784f4b158b3c0f7a282b04bd816887
eb13ba873881abd5e15af784756a61af635e665e
a67da70dc0955580665f5444f318b92e69a3c272
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