linux-kernel - Re: [PATCH 6/8] RCU, workqueue: Implement rcu

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Message-ID: <20180319164523.GB519464@devbig577.frc2.facebook.com>
Date:   Mon, 19 Mar 2018 09:45:23 -0700
From:   Tejun Heo <tj@...nel.org>
To:     Lai Jiangshan <jiangshanlai+lkml@...il.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jann Horn <jannh@...gle.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Benjamin LaHaise <bcrl@...ck.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kent Overstreet <kent.overstreet@...il.com>,
        security@...nel.org, LKML <linux-kernel@...r.kernel.org>,
        kernel-team@...com
Subject: Re: [PATCH 6/8] RCU, workqueue: Implement rcu_work

Hello, Lai.

On Fri, Mar 16, 2018 at 02:01:35PM +0800, Lai Jiangshan wrote:
> > +bool flush_rcu_work(struct rcu_work *rwork)
> > +{
> > +       if (test_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&rwork->work))) {
> > +               rcu_barrier();
> > +               flush_work(&rwork->work);
> > +               return true;
> 
> A possible tiny improvement: check if it was already queued on wq.
> For example:
> 
>        if (test_bit(WORK_STRUCT_PENDING_BIT, work_data_bits(&rwork->work))) {
>                if (!flush_work(&rwork->work)) {
>                       rcu_barrier();
>                       flush_work(&rwork->work);
>                }
>                return true;

But this breaks the guarantee that flush_work waits for the latest
queueing instance.  Please consider the following scenario.


 1. rcu-work is queued
 2. rcu-work starts executing
				3. rcu-work is queued again
				4. rcu-work is flushed
 5. execution finishes
				6. flush finishes
				7. execution finishes

6 should happen after 7 but it didn't.

Thanks.

-- 
tejun