linux-kernel - Re: is printk() safe within a timekeeper

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] [day] [month] [year] [list]

Message-ID: <5334C71B.1090506@linaro.org>
Date:	Thu, 27 Mar 2014 17:49:31 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Jiri Bohac <jbohac@...e.cz>
CC:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: is printk() safe within a timekeeper_seq write section?

On 03/12/2014 02:21 AM, Jiri Bohac wrote:
> On Tue, Mar 11, 2014 at 02:54:13PM -0700, John Stultz wrote:
>> Ok, so a generic solution is probably not going to be worth it then. My
>> thought was that since we do a very limited amount of informational
>> printks in the timekeeping code, we can be fairly safe delaying the
>> print-out until we drop the locks.
>>
>> For timekeeping, its really 4 call sites:
>> * invalid inject_sleep_time deltas
>> * > 11% clocksource freq adjustments
>> * insert leap second
>> * delete leap second
> I believe these last two were made safe by 
> commit ca4523cd (timekeeping: Shorten seq_count region). 
>
> write_seqcount_begin(&timekeeper_seq) is now done after the
> accumulate_nsecs_to_secs(tk) from where the printks are called.

So I started looking into deferring the printk with a small local
buffer, but I suddenly realized there are all these WARN_ON's around
which would likely have the same problem, no?

So I'm starting to doubt we can safely get away with a timekeeping
specific hack to defer the printk. :(

thanks
-john




--
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/