Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1753539AbbKQNc1 (ORCPT <rfc822;w@1wt.eu>);
	Tue, 17 Nov 2015 08:32:27 -0500
Received: from smtprelay.synopsys.com ([198.182.60.111]:39006 "EHLO
	smtprelay.synopsys.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1752811AbbKQNcX (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Tue, 17 Nov 2015 08:32:23 -0500
Subject: Re: [PATCH v2 18/19] ARC: [plat-eznps] replace sync with proper cpu
 barrier
To: Peter Zijlstra <peterz@infradead.org>
References: <1446297327-16298-1-git-send-email-noamc@ezchip.com>
 <1446893557-29748-19-git-send-email-noamc@ezchip.com>
 <564B0BB1.8080709@synopsys.com>
 <20151117112343.GW3816@twins.programming.kicks-ass.net>
 <564B1182.7050107@synopsys.com>
 <20151117122208.GX3816@twins.programming.kicks-ass.net>
 <564B1F74.6090709@synopsys.com>
 <20151117124421.GY17308@twins.programming.kicks-ass.net>
CC: <gilf@ezchip.com>, <talz@ezchip.com>, <linux-kernel@vger.kernel.org>,
        <cmetcalf@ezchip.com>, Noam Camus <noamc@ezchip.com>,
        <linux-snps-arc@lists.infradead.org>
Newsgroups: gmane.linux.kernel.arc,gmane.linux.kernel
From: Vineet Gupta <Vineet.Gupta1@synopsys.com>
Message-ID: <564B2C50.3080401@synopsys.com>
Date: Tue, 17 Nov 2015 19:02:00 +0530
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101
 Thunderbird/38.3.0
MIME-Version: 1.0
In-Reply-To: <20151117124421.GY17308@twins.programming.kicks-ass.net>
Content-Type: text/plain; charset="windows-1252"
Content-Transfer-Encoding: 7bit
X-Originating-IP: [10.12.197.182]
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1785
Lines: 38

On Tuesday 17 November 2015 06:14 PM, Peter Zijlstra wrote:
> 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 :-)

Indeed, loop for say 32 times at max or some such.


> 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?

Yeah, although I need to ensure if arch code needs to check that. Plus the
unwinder is also used for things like ps wchan, /proc/<pid>/stack, crash dump etc.

>> 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.

For perf case, but there are other uses of unwinder as described above.

So in light of above, do u think that the cond_resched() + signal_pending check is
not needed and the bail if over N iters will be fine !
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/