Re: workqueue thing

Linus Torvalds
2009-12-18T15:34:21Z

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).

Linus