linux-kernel - Re: [PATCH] workqueue/lockdep: Explicitly initialize wq

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Date:   Thu, 17 Aug 2017 06:26:35 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Byungchul Park <byungchul.park@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] workqueue/lockdep: Explicitly initialize
 wq_barrier::done::map

On Thu, Aug 17, 2017 at 05:46:12PM +0800, Boqun Feng wrote:
> With CROSSRELEASE feature introduced, such a potential deadlock is
> reported:
> 
> > Worker A : acquired of wfc.work -> wait for cpu_hotplug_lock to be released
> > Task   B : acquired of cpu_hotplug_lock -> wait for lock#3 to be released
> > Task   C : acquired of lock#3 -> wait for completion of barr->done
> > (Task C is in lru_add_drain_all_cpuslocked())
> > Worker D : wait for wfc.work to be released -> will complete barr->done
> 
> However, given this very case, such a dead lock could not happen because
> Task C's barr->done and Worker D's barr->done can not be the same
> instance. So this is a false positive.
> 
> The reason of this false positive is we initialize all wq_barrier::done
> at insert_wq_barrier() via init_completion(), which makes them belong to
> the same lock class, therefore, impossible circles are reported.
> 
> To fix this, explicitly initialize the lockdep map for wq_barrier::done
> in insert_wq_barrier(), so that the lock class key of wq_barrier::done
> is a subkey of the corresponding work_struct, as a result we won't build
> a dependency between a wq_barrier with a unrelated work, and we can
> differ wq barriers based on the related works, so the false positive
> above is avoided.
> 
> Also define the empty lockdep_init_map_crosslock() for !CROSSRELEASE
> to make the code simple and away from unnecessary #ifdefs.
> 
> Reported-by: Ingo Molnar <mingo@...nel.org>
> Signed-off-by: Boqun Feng <boqun.feng@...il.com>
> Cc: Byungchul Park <byungchul.park@....com>

Acked-by: Tejun Heo <tj@...nel.org>

Thanks.

-- 
tejun