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SubjectRe: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
On Thu, Aug 31, 2017 at 10:34:53AM +0200, Peter Zijlstra wrote:
> On Thu, Aug 31, 2017 at 05:15:01PM +0900, Byungchul Park wrote:
> > It's not important. Ok, check the following, instead:
> >
> > context X context Y
> > --------- ---------
> > wait_for_completion(C)
> > acquire(A)
> > release(A)
> > process_one_work()
> > acquire(B)
> > release(B)
> > work->fn()
> > complete(C)
> >
> > We don't need to lose C->A and C->B dependencies unnecessarily.
>
> I really can't be arsed about them. Its really only the first few works
> that will retain that dependency anyway, even if you were to retain
> them.

Wrong.

Every 'work' doing complete() for different classes of completion
variable suffers from losing valuable dependencies, every time, not
first few ones.

Remind we are talking about dependencies wrt cross-lock, not between
_holding_ locks. If you invalidate xhlock whenever work->fn(), we cannot
build dependencies like C->A and C->B every time. Right?

> All of that is contained in kernel/kthread and kernel/workqueue and can
> be audited if needed. Its a very limited amount of code.

I mean, doing it automatically w/o additional overhead is better than
considering the limited amount of code manually every time changing
kernel code. Do as you please.

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