Subject: net/sched: latent livelock in dev_deactivate_many() due to yield() usage

From: Mike Galbraith <[email-masked]>
Date: Sun, 02 Apr 2017 06:28:41 +0200

Greetings network wizards,

Quoting kernel/sched/core.c:
/**
 * yield - yield the current processor to other threads.
 *
 * 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!
 */

Livelock can be triggered by setting kworkers to SCHED_FIFO, then
suspend/resume.. you come back from sleepy-land with a spinning
kworker.  For whatever reason, I can only do that with an enterprise
like config, my standard config refuses to play, but no matter, it's
"Typical broken usage".

(yield() should be rendered dead)

	-Mike