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

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Message-Id: <20170630125020.GU2393@linux.vnet.ibm.com>
Date:   Fri, 30 Jun 2017 05:50:20 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Oleg Nesterov <oleg@...hat.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 Fri, Jun 30, 2017 at 01:04:45PM +0200, Oleg Nesterov wrote:
> On 06/29, Paul E. McKenney wrote:
> >
> > --- a/kernel/task_work.c
> > +++ b/kernel/task_work.c
> > @@ -109,7 +109,8 @@ void task_work_run(void)
> >  		 * the first entry == work, cmpxchg(task_works) should
> >  		 * fail, but it can play with *work and other entries.
> >  		 */
> > -		raw_spin_unlock_wait(&task->pi_lock);
> > +		raw_spin_lock(&task->pi_lock);
> > +		raw_spin_unlock(&task->pi_lock);
> 
> Well, bit the you need spin_lock_irq(). And this is one of the reasons
> why I personally think unlock_wait have some sense...

Good catch, and I clearly need to double-check the other commits for
any need to disable interrupts.  Anyway, like this, with the addition
of a flags variable, correct?

> > +		raw_spin_lock_irq(&task->pi_lock);
> > +		raw_spin_unlock_irq(&task->pi_lock);

I agree that the spin_unlock_wait() implementations would avoid the
deadlock with an acquisition from an interrupt handler, while also
avoiding the need to momentarily disable interrupts.  The ->pi_lock is
a per-task lock, so I am assuming (perhaps naively) that contention is
not a problem.  So is the overhead of interrupt disabling likely to be
noticeable here?

							Thanx, Paul