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From: Bart Van Assche <bvanassche@acm.org>
To: Johannes Berg <johannes@sipsolutions.net>, Tejun Heo <tj@kernel.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Christoph Hellwig <hch@lst.de>, Sagi Grimberg <sagi@grimberg.me>,
	"tytso@mit.edu" <tytso@mit.edu>
Subject: Re: [PATCH 2/3] kernel/workqueue: Surround work execution with shared lock annotations
Date: Thu, 25 Oct 2018 10:22:42 -0700	[thread overview]
Message-ID: <1540488162.66186.35.camel@acm.org> (raw)
In-Reply-To: <256720b373cf539052d79ce3051214140820d696.camel@sipsolutions.net>

On Thu, 2018-10-25 at 18:53 +0200, Johannes Berg wrote:
> On Thu, 2018-10-25 at 15:05 +0000, Bart Van Assche wrote:
> > Surround execution of work with a shared lockdep annotation because multiple
> > work items associated with a work queue may execute concurrently.
> 
> Hmm. So, I'm not really entirely sure of the semantics here, but I fail
> to see how "may execute concurrently" means "can be taken recursively"?
> 
> After all, if they execute concurrently, that's in a different thread,
> right? So each thread is really just doing something with this work. It
> may not match mutex semantics in how mutexes would lock each other out
> and prevent concurrency, but I don't think that matters to lockdep at
> all.
> 
> In fact, I'm not sure this actually changes anything, since you can't
> really execute a work struct while executing one already?
> 
> What's this intended to change? I currently don't see how lockdep's
> behaviour would differ with read==1, unless you actually tried to do
> recursive locking, which isn't really possible?
> 
> Or perhaps this is actually the right change for the issue described in
> patch 1, where a work struct flushes another work on the same wq, and
> that causes recursion of sorts? But that recursion should only happen if
> the workqueues is actually marked as ordered, in which case it *is* in
> fact wrong?

How about modifying the wq->lockdep_map annotations only and not touching the
work->lockdep_map annotations? My comment about concurrency in the patch
description refers to a multithreaded workqueue executing multiple different
work items concurrently. I am aware that great care has been taken in the
workqueue implementation to ensure that each work item is executed by exactly
one worker.

Bart.

  reply	other threads:[~2018-10-25 17:22 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-10-25 15:05 [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Bart Van Assche
2018-10-25 15:05 ` [PATCH 1/3] kernel/workqueue: Remove lockdep annotation from __flush_work() Bart Van Assche
2018-10-25 15:31   ` Johannes Berg
2018-10-25 15:57     ` Johannes Berg
2018-10-25 16:01     ` Bart Van Assche
2018-10-25 15:05 ` [PATCH 2/3] kernel/workqueue: Surround work execution with shared lock annotations Bart Van Assche
2018-10-25 16:53   ` Johannes Berg
2018-10-25 17:22     ` Bart Van Assche [this message]
2018-10-25 19:17       ` Johannes Berg
2018-10-25 15:05 ` [PATCH 3/3] kernel/workqueue: Suppress a false positive lockdep complaint Bart Van Assche
2018-10-25 15:34   ` Johannes Berg
2018-10-25 15:55     ` Bart Van Assche
2018-10-25 19:59       ` Johannes Berg
2018-10-25 20:21         ` Theodore Y. Ts'o
2018-10-25 20:26           ` Johannes Berg
2018-10-25 15:36   ` Tejun Heo
2018-10-25 15:37     ` Tejun Heo
2018-10-25 20:13     ` Johannes Berg
2018-10-25 15:40   ` Theodore Y. Ts'o
2018-10-25 17:02   ` Johannes Berg
2018-10-25 17:11     ` Bart Van Assche
2018-10-25 19:51       ` Johannes Berg
2018-10-25 20:39         ` Bart Van Assche
2018-10-25 20:47           ` Johannes Berg
2018-10-25 15:27 ` [PATCH 0/3] Suppress false positives triggered by workqueue lockdep annotations Johannes Berg
2018-10-25 15:47   ` Bart Van Assche

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