linux-kernel - Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

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, 16 Jun 2008 08:22:49 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	Tom Zanussi <tzanussi@...il.com>, penberg@...helsinki.fi,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	righi.andrea@...il.com
Subject: Re: [PATCH 2/3] relay: Fix race condition which occurs when
	reading across CPUs.

* Eduard - Gabriel Munteanu (eduard.munteanu@...ux360.ro) wrote:
> On Fri, 13 Jun 2008 23:26:41 -0500
> Tom Zanussi <tzanussi@...il.com> wrote:
> 
> > Alternatively, you could get rid of the problem by making sure CPU0
> > never reads CPU1's data, by having the userspace reader use per-cpu
> > threads and using sched_setaffinity() to pin each thread to a given
> > cpu. See for example, the blktrace code, which does this.
> 
> Yes, and performance-wise this is better. Though I'm not sure setting
> affinity is 100% safe. Will the thread be migrated soon enough, so we
> don't read cross-CPU? The point is I'm not sure how hard this is
> enforced.
> 
> However, I suggest this patch should go in, for two reasons:
> 1. It provides expected behavior in any such situation.
> 2. It adds (almost) no overhead when used in conjuction with setting CPU
> affinity. When the writer acquires the spinlock, it does not busy-wait,
> so the spinlock just disables IRQs (relay_write()).
>  

Hi Eduard,

Two objections against this. First, taking a spinlock _is_ slow in SMP
because it involves synchronized atomic operations.

Second, Adding a spinlock to the relay write side is bad since it
opens the door to deadly embrace between a trap handler and normal
kernel code both running tracing code.

Unless really-really needed, which does not seem to be the case, I would
strongly recommend not merging this patch.

Mathieu


> > Actually, in a few days or so I'm planning on releasing the first cut
> > of a library that makes this and all the rest of the nice blktrace
> > userspace code available to other tracing applications, not just
> > blktrace.  Hopefully it would be something that you'd be able to use
> > for kmemtrace as well; in that case, you'd just use the library and
> > not have to worry about these details.
> 
> Sounds good. Thanks for letting me know.
> 
> 
> 	Eduard
> 

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
--
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/