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

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Message-ID: <1331056466.11248.327.camel@twins>
Date:	Tue, 06 Mar 2012 18:54:26 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Joe Perches <joe@...ches.com>
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 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!
  */
 void __sched yield(void)
 {

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