linux-kernel - Re: softlockups in multi_cpu

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Date:	Fri, 06 Mar 2015 14:15:37 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jason Low <jason.low2@...com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...emonkey.org.uk>,
	Ming Lei <ming.lei@...onical.com>
Subject: Re: softlockups in multi_cpu_stop

On Fri, 2015-03-06 at 13:12 -0800, Jason Low wrote:
> In owner_running() there are 2 conditions that would make it return
> false: if the owner changed or if the owner is not running. However,
> that patch continues spinning if there is a "new owner" but it does not
> take into account that we may want to stop spinning if the owner is not
> running (due to getting rescheduled).

So you're rationale is that we're missing this need_resched:

	while (owner_running(sem, owner)) {
		/* abort spinning when need_resched */
		if (need_resched()) {
			rcu_read_unlock();
			return false;
		}
	}

Because the owner_running() would return false, right? Yeah that makes
sense, as missing a resched is a bug, as opposed to our heuristics being
so painfully off.

Sasha, Ming (Cc'ed), does this address the issues you guys are seeing?

Thanks,
Davidlohr

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