Re: [PATCH 4/4] lockdep: Fix workqueue crossrelease annotation
From: Byungchul Park
Date: Tue Aug 29 2017 - 02:46:45 EST
On Wed, Aug 23, 2017 at 01:58:47PM +0200, Peter Zijlstra wrote:
> The new completion/crossrelease annotations interact unfavourable with
> the extant flush_work()/flush_workqueue() annotations.
>
> The problem is that when a single work class does:
>
> wait_for_completion(&C)
>
> and
>
> complete(&C)
>
> in different executions, we'll build dependencies like:
>
> lock_map_acquire(W)
> complete_acquire(C)
>
> and
>
> lock_map_acquire(W)
> complete_release(C)
>
> which results in the dependency chain: W->C->W, which lockdep thinks
> spells deadlock, even though there is no deadlock potential since
> works are ran concurrently.
>
> One possibility would be to change the work 'lock' to recursive-read,
> but that would mean hitting a lockdep limitation on recursive locks.
> Also, unconditinoally switching to recursive-read here would fail to
> detect the actual deadlock on single-threaded workqueues, which do
> have a problem with this.
>
> For now, forcefully disregard these locks for crossrelease.
Eventually, you pushed this patch to tip tree without any comment.
I don't really understand you.
How does a maintainer choose a very work-around method and avoid
problems rather than fix a root cause? I am very disappointed.
But, I have nothing to do against your will.