linux-kernel - Re: [RFC PATCH] checkpatch: Warn on use of yield()

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Date:	Tue, 6 Mar 2012 17:14:16 +0400
From:	Glauber Costa <glommer@...allels.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Joe Perches <joe@...ches.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Miao Xie <miaox@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>, Theodore Ts'o <tytso@....edu>
Subject: Re: [RFC PATCH] checkpatch: Warn on use of yield()

On 03/06/2012 04:45 PM, Peter Zijlstra wrote:
> On Mon, 2012-03-05 at 18:01 -0800, Joe Perches wrote:
>
>> +# check for use of yield()
>> +		if ($line =~ /\byield\s*\(\s*\)/ {
>> +			WARN("YIELD",
>> +			     "yield() is deprecated, consider cpu_relax()\n"  . $herecurr);
>> +		}
>
> Its not deprecated as such, its just a very dangerous and ill considered
> API.
>
> cpu_relax() is not a good substitute suggestion in that its still a busy
> wait and prone to much of the same problems.
>
> The case at hand was a life-lock due to expecting that yield() would run
> another process which it needed in order to complete. Yield() does not
> provide that guarantee.
>
> Looking at fs/ext4/mballoc.c, we have this gem:
>
>
> 		/*
>                   * Yield the CPU here so that we don't get soft lockup
>                   * in non preempt case.
>                   */
>                  yield();
>
> This is of course complete crap as well.. I suspect they want
> cond_resched() there. And:
>
>                          /* let others to free the space */
>                          yield();
>
> Like said, yield() doesn't guarantee anything like running anybody else,
> does it rely on that? Or is it optimistic?
>
> Another fun user:
>
> void tasklet_kill(struct tasklet_struct *t)
> {
>          if (in_interrupt())
>                  printk("Attempt to kill tasklet from interrupt\n");
>
>          while (test_and_set_bit(TASKLET_STATE_SCHED,&t->state)) {
>                  do {
>                          yield();
>                  } while (test_bit(TASKLET_STATE_SCHED,&t->state));
>          }
>          tasklet_unlock_wait(t);
>          clear_bit(TASKLET_STATE_SCHED,&t->state);
> }
>
> The only reason that doesn't explode is because running tasklets is
> non-preemptible, However since they're non-preemptible they shouldn't
> run long and you might as well busy spin. If they can run long, yield()
> isn't your biggest problem.
>
> mm/memory_hotplug.c has two yield() calls in offline_pages() and I've no
> idea what they're trying to achieve.
>
> But really, yield() is basically _always_ the wrong thing. The right
> thing can be:
>
>   cond_resched(); wait_event(); or something entirely different.
>
> So instead of suggesting an alternative, I would suggest thinking about
> the actual problem in order to avoid the non-thinking solutions the
> checkpatch brigade is so overly fond of :/
>
> Maybe something like:
>
>   "yield() is dangerous and wrong, rework your code to not use it."
>
> That at least requires some sort of thinking and doesn't suggest blind
> substitution.
>

Can't we point people to some Documentation file that explains the 
alternatives?
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