linux-kernel - Re: [PATCH] sched: Fix lost reschedule in __cond

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Date:	Sun, 14 Dec 2014 11:50:20 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...nel.org>, Dave Jones <davej@...hat.com>,
	Chris Mason <clm@...com>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Dâniel Fraga <fragabr@...il.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Fix lost reschedule in __cond_resched()

On Sun, Dec 14, 2014 at 10:04 AM, Frederic Weisbecker
<fweisbec@...il.com> wrote:
>
> Such as:

So I like your patch, but quite frankly, can we go one step further?

Look at the callers of __schedule().

EVERY SINGLE ONE now has that loop around it that goes along the lines of

   do {
      .. disable preemption somehow ..
      __schedule();
      ...enable preemption without scheduling ..
   } while (need_resced());

except for one - the regular "schedule()" function.

Furthermore, look inside __schedule() itself: it has the same loop,
except with a count of one.

So I would suggest going the extra mile, and
 - remove the loop from __schedule() itself
 - add the same loop as everywhere else to "schedule()"

IOW, just make this "you have to loop and disable preemption" thing be
a rule that __schedule() can depend on.

                 Linus
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