Messages in this thread | | | Date | Sun, 27 Aug 2006 12:41:16 +0530 | From | Dipankar Sarma <> | Subject | Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking. |
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On Sat, Aug 26, 2006 at 11:46:18PM -0700, Andrew Morton wrote: > On Sun, 27 Aug 2006 11:41:55 +0530 > Dipankar Sarma <dipankar@in.ibm.com> wrote: > > > 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. > > Yes you do. Please, read _cpu_up(), _cpu_down() and the example in > workqueue_cpu_callback(). It's really very simple.
What are you talking about here ? That is the write side. You are *not* supposed to do lock_cpu_hotplug() in cpu callbacks paths AFAICT. If someone does it (like cpufreq did), it is just *wrong*.
> > With multiple low-level subsystems > > needing it, it also becomes difficult to work out the lock > > hierarchies. > > That'll matter if we do crappy code. Let's not do that?
I am talking about readsides here - you read cpu_online_map and block then reuse the map and make some calls to another subsystem that may again do a similar read-side cpu_hotplug lock. I suspect that it hard to get rid of all possible dependencies.
> > > 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 code's already racy and I don't think anyone has reported a > cpufreq-vs-hotplug race.
Do cpu hotplugs in a one cpu and change frequencies in another - I think Gautham has a script to reproduce this. Besides lockdep apparently complains about it -
http://marc.theaimsgroup.com/?l=linux-kernel&m=115359728428432&w=2
> > The right thing to do would be to > > do an audit and clean up the bad lock_cpu_hotplug() calls. > > No, that won't fix it. For example, take a look at all the *callers* of > cpufreq_update_policy(). AFAICT they're all buggy. Fiddling with the > existing lock_cpu_hotplug() sites won't fix that. (Possibly this > particular problem can be fixed by checking that the relevant CPU is still > online after the appropriate locking has been taken - dunno). > > It needs to be ripped out and some understanding, thought and design should > be applied to the problem.
Really, the hotplug locking rules are fairly simple-
1. If you are in cpu hotplug callback path, don't take any lock.
2. If you are in a non-hotplug path reading cpu_online_map and you don't block, you just disable preemption and you are safe from hotplug.
3. If you are in a non-hotplug path and you use cpu_online_map and you *really* need to block, you use lock_cpu_hotplug() or cpu_hotplug_disable whatever it is called.
Is this too difficult for people to follow ?
> > > People > > seem to have just got lazy with lock_cpu_hotplug(). > > That's because lock_cpu_hotplug() purports to be some magical thing which > makes all your troubles go away.
No it doesn't. Perhaps we should just document the rules better and put some static checks for people to get it right.
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