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

[Mathieu Desnoyers <compudj@krystal.dyndns.org>]

* Eduard - Gabriel Munteanu (eduard.munteanu@linux360.ro) wrote:
> On Fri, 13 Jun 2008 23:26:41 -0500
> Tom Zanussi <tzanussi@gmail.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