linux-kernel - Re: [debug patch] printk: Add a printk killswitch to robustify NMI watchdog messages

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 06 Jun 2011 18:45:23 +0200
From:	Arne Jansen <lists@...-jansens.de>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	mingo@...hat.com, hpa@...or.com, linux-kernel@...r.kernel.org,
	efault@....de, npiggin@...nel.dk, akpm@...ux-foundation.org,
	frank.rowand@...sony.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [debug patch] printk: Add a printk killswitch to robustify NMI
 watchdog messages

On 06.06.2011 18:38, Arne Jansen wrote:
> On 06.06.2011 18:17, Ingo Molnar wrote:
>>
>> * Peter Zijlstra<peterz@...radead.org> wrote:
>>
>>> On Mon, 2011-06-06 at 18:08 +0200, Ingo Molnar wrote:
>>>> * Peter Zijlstra<peterz@...radead.org> wrote:
>>>>
>>>>> On Mon, 2011-06-06 at 17:52 +0200, Ingo Molnar wrote:
>>>>>> * Peter Zijlstra<peterz@...radead.org> wrote:
>>>>>>
>>>>>>> Needs more staring at, preferably by someone who actually
>>>>>>> understands that horrid mess :/ Also, this all still doesn't make
>>>>>>> printk() work reliably while holding rq->lock.
>>>>>>
>>>>>> So, what about my suggestion to just *remove* the wakeup from there
>>>>>> and use the deferred wakeup mechanism that klogd uses.
>>>>>>
>>>>>> That would make printk() *visibly* more robust in practice.
>>>>>
>>>>> That's currently done from the jiffy tick, do you want to effectively
>>>>> delay releasing the console_sem for the better part of a jiffy?
>>>>
>>>> Yes, and we already do it in some other circumstances.
>>>
>>> We do?
>>
>> Yes, see the whole printk_pending logic, it delays:
>>
>> wake_up_interruptible(&log_wait);
>>
>> to the next jiffies tick.
>>
>>>> Can you see
>>>> any problem with that? klogd is an utter slowpath anyway.
>>>
>>> but console_sem isn't klogd. We delay klogd and that's perfectly
>>> fine, but afaict we don't delay console_sem.
>>
>> But console_sem is really a similar special case as klogd. See, it's
>> about a *printk*. That's rare by definition.
>>
>> If someone on the console sees it he'll be startled by at least 10
>> msecs ;-) So delaying the wakeup to the next jiffy really fits into
>> the same approach as we already do with&log_wait, hm?
>
> As long as it doesn't scramble the order of the messages, the delay
> imho doesn't matter even in very printk-heavy debugging sessions.

And, as important, doesn't reduce the throughput of printk. Having only
100 wakeups/s sounds like the throughput is limited to 100xsizeof(ring 
buffer).

>
>>
>> This would solve a real nightmare that has plagued us ever since
>> printk() has done wakeups directly - i.e. like forever.
>>
>> Thanks,
>>
>> Ingo
>
> --
> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> the body of a message to majordomo@...r.kernel.org
> More majordomo info at http://vger.kernel.org/majordomo-info.html
> Please read the FAQ at http://www.tux.org/lkml/

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/