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

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Message-ID: <20200528140613.GK30374@kadam>
Date:   Thu, 28 May 2020 17:06:13 +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 09:27:03PM +0800, Lai Jiangshan wrote:
> On Thu, May 28, 2020 at 8:27 PM Dan Carpenter <dan.carpenter@...cle.com> wrote:
> >
> > 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.
> 
> wq owns the ultimate or permanent references to itself by
> owning references to wq->numa_pwq_tbl[node], wq->dfl_pwq.
> The pwq's references keep the pwq in wq->pwqs.
> 
> Only destroy_workqueue() can release these ultimate references
> and then (after maybe a period of time) deplete the wq->pwqs
> finally and then free itself in rcu callback.
> 
> Actually, in short, we don't need the care about the above
> detail. The responsibility to free rescuer is on
> destroy_workqueue(), and since it does the free early,
> it doesn't need to do it again later.
> 
> e2dca7adff8f moved the free of rescuer into rcu callback,
> and I didn't check all the changes between then and now.
> But at least now, the rescuer is not accessed in rcu mananer,
> so we don't need to free it in rcu mananer.
> 

Ah...  Thanks for the explanation!

regards,
dan carpenter