linux-kernel - Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1) instead of yield() to wait for outstanding qdisc

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Message-ID: <1394392160.15968.13.camel@deadeye.wl.decadent.org.uk>
Date:	Sun, 09 Mar 2014 19:09:20 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	David Miller <davem@...emloft.net>
Cc:	mkl@...gutronix.de, linux-rt-users@...r.kernel.org,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	kernel@...gutronix.de
Subject: Re: [PATCH] net: sched: dev_deactivate_many(): use msleep(1)
 instead of yield() to wait for outstanding qdisc_run calls

On Thu, 2014-03-06 at 16:06 -0500, David Miller wrote:
> From: Marc Kleine-Budde <mkl@...gutronix.de>
> Date: Wed,  5 Mar 2014 00:49:47 +0100
> 
> > @@ -839,7 +839,7 @@ void dev_deactivate_many(struct list_head *head)
> >       /* Wait for outstanding qdisc_run calls. */
> >       list_for_each_entry(dev, head, unreg_list)
> >               while (some_qdisc_is_busy(dev))
> > -                     yield();
> > +                     msleep(1)
> >  }
> 
> I don't understand this.
> 
> yield() should really _mean_ yield.
> 
> The intent of a yield() call, like this one here, is unambiguously
> that the current thread cannot do anything until some other thread
> gets onto the cpu and makes forward progress.
>
> Therefore it should allow lower priority threads to run, not just
> equal or higher priority ones.

Until when?

yield() is not a sensible operation in a preemptive multitasking system,
regardless of RT.

Ben.

-- 
Ben Hutchings
I say we take off; nuke the site from orbit.  It's the only way to be sure.

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