netdev - Re: [PATCH RFC 02/26] task_work: Replace spin_unlock

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Message-ID: <20170630192123.GA8471@redhat.com>
Date:   Fri, 30 Jun 2017 21:21:23 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org, netfilter-devel@...r.kernel.org,
        netdev@...r.kernel.org, akpm@...ux-foundation.org,
        mingo@...hat.com, dave@...olabs.net, manfred@...orfullife.com,
        tj@...nel.org, arnd@...db.de, linux-arch@...r.kernel.org,
        will.deacon@....com, peterz@...radead.org,
        stern@...land.harvard.edu, parri.andrea@...il.com,
        torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with
 lock/unlock pair

On 06/30, Paul E. McKenney wrote:
>
> On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote:
> >
> > I do not think the overhead will be noticeable in this particular case.
> >
> > But I am not sure I understand why do we want to unlock_wait. Yes I agree,
                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

if it was not clear, I tried to say "why do we want to _remove_ unlock_wait".

> > it has some problems, but still...
> >
> > The code above looks strange for me. If we are going to repeat this pattern
> > the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;)
> >
> > If not, we should probably change this code more:
>
> This looks -much- better than my patch!  May I have your Signed-off-by?

Only if you promise to replace all RCU flavors with a single simple implementation
based on rwlock ;)

Seriously, of course I won't argue, and it seems that nobody except me likes
this primitive, but to me spin_unlock_wait() looks like synchronize_rcu(() and
sometimes it makes sense.

Including this particular case. task_work_run() is going to flush/destroy the
->task_works list, so it needs to wait until all currently executing "readers"
(task_work_cancel()'s which have started before ->task_works was updated) have
completed.

Oleg.