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Date: Sun, 27 Aug 2006 11:41:55 +0530 From: Dipankar Sarma <dipankar@...ibm.com> To: Andrew Morton <akpm@...l.org> Cc: Linus Torvalds <torvalds@...l.org>, Dave Jones <davej@...hat.com>, ego@...ibm.com, rusty@...tcorp.com.au, linux-kernel@...r.kernel.org, arjan@...el.linux.com, mingo@...e.hu, vatsa@...ibm.com, ashok.raj@...el.com Subject: Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking. On Sat, Aug 26, 2006 at 03:04:22PM -0700, Andrew Morton wrote: > On Sat, 26 Aug 2006 14:09:55 -0700 (PDT) > Linus Torvalds <torvalds@...l.org> wrote: > > > I definitely want to have this fixed, and Gautham's patches look like a > > good thing to me, but the "trying to fix up the current mess" part was > > really about trying to get 2.6.18 in a mostly working state rather than > > anything else. I think it's been too late to try to actually _fix_ it for > > 2.6.18 for a long time already. > > > > So my reaction is that this redesign should go in asap after 2.6.18, > > unless people feel strongly that the current locking has so many bugs that > > people can actually _hit_ in practice that it's better to go for the > > redesign early. > > It certainly needs a redesign. A new sort of lock which makes it appear to > work won't fix races like: > > int cpufreq_update_policy(unsigned int cpu) > { > struct cpufreq_policy *data = cpufreq_cpu_get(cpu); > > ... > > lock_cpu_hotplug(); > The problem with cpufreq was that it used lock_cpu_hotplug() in some common routines because it was needed in some paths and then also called the same routines from the CPU hotplug callback path. That is easily fixable and Gautham's patch 1/4 does exactly that. One thing I have privately suggested to Gautham is to do an audit of bad lock_cpu_hotplug() uses. Now coming to the read-side of lock_cpu_hotplug() - cpu hotplug is a very special asynchronous event. You cannot protect against it using your own subsystem lock because you don't control access to cpu_online_map. With multiple low-level subsystems needing it, it also becomes difficult to work out the lock hierarchies. The right way to do this is what Gautham and Ingo are discussing - a scalable rw semaphore type lock that allows recursive readers. > > I rather doubt that anyone will be hitting the races in practice. I'd > recommend simply removing all the lock_cpu_hotplug() calls for 2.6.18. I don't think that is a good idea. The right thing to do would be to do an audit and clean up the bad lock_cpu_hotplug() calls. People seem to have just got lazy with lock_cpu_hotplug(). Thanks Dipankar - 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/
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