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

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Message-Id: <20170630200448.GG2393@linux.vnet.ibm.com>
Date:   Fri, 30 Jun 2017 13:04:48 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     Oleg Nesterov <oleg@...hat.com>, 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, 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 Fri, Jun 30, 2017 at 03:50:33PM -0400, Alan Stern wrote:
> On Fri, 30 Jun 2017, Oleg Nesterov wrote:
> 
> > 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.
> 
> If it looks like synchronize_rcu(), why not actually use 
> synchronize_rcu()?

My guess is that the latencies of synchronize_rcu() don't suit his needs.
When the lock is not held, spin_unlock_wait() is quite fast, even
compared to expedited grace periods.

							Thanx, Paul

> Alan Stern
> 
> > 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.
>