linux-kernel - Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu barrier

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Message-ID: <20151117124421.GY17308@twins.programming.kicks-ass.net>
Date:	Tue, 17 Nov 2015 13:44:21 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
Cc:	gilf@...hip.com, talz@...hip.com, linux-kernel@...r.kernel.org,
	cmetcalf@...hip.com, Noam Camus <noamc@...hip.com>,
	linux-snps-arc@...ts.infradead.org
Subject: Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu
 barrier

On Tue, Nov 17, 2015 at 06:07:08PM +0530, Vineet Gupta wrote:
> On Tuesday 17 November 2015 05:52 PM, Peter Zijlstra wrote:
> >> > BTW since we are on the topic we have this loop in stack unwinder which can
> >> > potentially cause RCU stalls, actual lockups etc. I was planning to add the
> >> > following - does that seem fine to you.
> > Worries me more than anything. How could you get stuck in there?
> 
> No we not getting stuck in there - but this has potential to - if say unwind info
> were corrupt (not seen that ever though).

Better put in a failsafe for that anyway, just out of sheer paranoia :-)

You should never report more than PERF_MAX_STACK_DEPTH thingies anyway,
so once you've done that many loops, you're good to bail, right?

> The old code won't even respond to say a Ctrl+C if it were stuck !
> Plus the reschedule there will keeps sched happy when say unraveling deep stack
> frames with perf ?

You're likely to call this code from interrupt/NMI context, there is no
^C or scheduling going to help you there.
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