linux-kernel - Re: softlockups in multi_cpu

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Message-ID: <1425697805.19505.92.camel@stgolabs.net>
Date:	Fri, 06 Mar 2015 19:10:05 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Ming Lei <ming.lei@...onical.com>
Cc:	Jason Low <jason.low2@...com>,
	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>
Subject: Re: softlockups in multi_cpu_stop

On Sat, 2015-03-07 at 10:55 +0800, Ming Lei wrote:
> On Sat, Mar 7, 2015 at 10:29 AM, Davidlohr Bueso <dave@...olabs.net> wrote:
> > On Fri, 2015-03-06 at 18:26 -0800, Davidlohr Bueso wrote:
> >> That's not what this is about. New lock _owners_ need to worry about
> >                                                       ^^^ make that "need not"
> 
> Sorry, could you explain a bit why new owner can't be scheduled
> out(on_cpu becomes zero)? If that is possible, it still can cause
> soft lockup like current problem.

Oh its not that it can't be scheduled out. The point is we don't care
what happens with the lock owner itself (new or not). We care about, and
the point of this discussion, how _other_ threads handle themselves when
trying to take that lock (a lock having an owner implies the lock is not
free, of course). So if a lock owner gets scheduled out... so what?
That's already taken into account by spinners.

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