linux-kernel - Re: 回复: [PATCH v5] workqueue: Remove unnecessary kfree() call in rcu_free

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Message-ID: <20200528122545.GP22511@kadam>
Date:   Thu, 28 May 2020 15:25:45 +0300
From:   Dan Carpenter <dan.carpenter@...cle.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     "Zhang, Qiang" <Qiang.Zhang@...driver.com>,
        Markus Elfring <markus.elfring@....de>,
        Tejun Heo <tj@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kernel-janitors@...r.kernel.org" <kernel-janitors@...r.kernel.org>
Subject: Re: 回复: [PATCH v5] workqueue:
 Remove unnecessary kfree() call in rcu_free_wq()

On Thu, May 28, 2020 at 08:08:06PM +0800, Lai Jiangshan wrote:
> On Thu, May 28, 2020 at 5:57 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >
> > Guys, the patch is wrong.  The kfree is harmless when this is called
> > from destroy_workqueue() and required when it's called from
> > pwq_unbound_release_workfn().  Lai Jiangshan already explained this
> > already.  Why are we still discussing this?
> >
> 
> I'm also confused why they have been debating about the changelog
> after the patch was queued. My statement was about "the patch is
> a correct cleanup, but the changelog is totally misleading".
> 
> destroy_workqueue(percpu_wq) -> rcu_free_wq()
> or
> destroy_workqueue(unbound_wq) -> put_pwq() ->
> pwq_unbound_release_workfn() -> rcu_free_wq()
> 
> So the patch is correct to me. Only can destroy_workqueue()
> lead to rcu_free_wq().

It looks like there are lots of paths which call put_pwq() and
put_pwq_unlocked().

  1168  static void pwq_dec_nr_in_flight(struct pool_workqueue *pwq, int color)
  1169  {
  1170          /* uncolored work items don't participate in flushing or nr_active */
  1171          if (color == WORK_NO_COLOR)
  1172                  goto out_put;
  1173  

We don't take an extra reference in this function.

  1200  out_put:
  1201          put_pwq(pwq);
  1202  }

I don't know this code well, so I will defer to your expertise if you
say it is correct.

> 
> Still, the kfree(NULL) is harmless. But it is cleaner
> to have the patch. But the changelog is wrong, even after
> the lengthened debating, and English is not my mother tongue,
> so I just looked on.

We have tried to tell Markus not to advise people about commit messages
but he doesn't listen.  He has discouraged some contributors.  :/

regards,
dan carpenter