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

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Message-ID: <1331056859.2140.7.camel@joe2Laptop>
Date:	Tue, 06 Mar 2012 10:00:59 -0800
From:	Joe Perches <joe@...ches.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	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 Tue, 2012-03-06 at 18:54 +0100, Peter Zijlstra wrote:
> On Tue, 2012-03-06 at 09:41 -0800, Joe Perches wrote:
> > Perhaps the kernel-doc comments in sched/core.c
> > should/could be expanded/updated. 
> 
> Something like this?
> 
> ---
>  kernel/sched/core.c |   20 ++++++++++++++++++--
>  1 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 2963fbb..a05a0f7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4577,8 +4577,24 @@ EXPORT_SYMBOL(__cond_resched_softirq);
>  /**
>   * yield - yield the current processor to other threads.
>   *
> - * This is a shortcut for kernel-space yielding - it marks the
> - * thread runnable and calls sys_sched_yield().
> + * Do not ever use this function, there's a 99% chance you're doing it wrong.
> + *
> + * The scheduler is at all times free to pick the calling task as the most
> + * eligible task to run, if removing the yield() call from your code breaks
> + * it, its already broken.
> + *
> + * Typical broken usage is:
> + *
> + * while (!event)
> + * 	yield();
> + *
> + * where one assumes that yield() will let 'the other' process run that will
> + * make event true. If the current task is a SCHED_FIFO task that will never
> + * happen. Never use yield() as a progress guarantee!!
> + *
> + * If you want to use yield() to wait for something, use wait_event().
> + * If you want to use yield() to be 'nice' for others, use cond_resched().
> + * If you still want to use yield(), do not!
>   */

Yes. I'll update the checkpatch message to say something like:

"Using yield() is generally wrong.  See yield() kernel-doc (sched/core.c)"


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