Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

[Andrew Morton <akpm@osdl.org>]

On Sun, 27 Aug 2006 12:41:16 +0530
Dipankar Sarma <dipankar@in.ibm.com> wrote:

> 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 ?

Did you look?  workqueue_mutex is used to protect per-cpu workqueue
resources.  The lock is taken prior to modification of per-cpu resources
and is released after their modification.  Very very simple.

> That is the write side. You are
> *not* supposed to do lock_cpu_hotplug() in cpu callbacks paths AFAICT. 

The workqueue code doesn't use lock_cpu_hotplug().

> 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.

Two unrelated subsystems which have both independent and interdependent CPU
hotplug locking requirements and neither of which can protect per-cpu
resources via preempt_disable()?  Sounds unlikely and undesirable.

> 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.

That rule is wrong.  The CPU_UP_PREPARE and CPU_DOWN_PREPARE notification
entrypoints are the logical place for a subsystem to lock any per-cpu resources
which another thread/cpu might presently be using.

> 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.

Sure.

> 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 ?

Apparently.  What's happening is that lock_cpu_hotplug() is seen as some
amazing thing which will prevent an *event* from occurring.

There's an old saying "lock data, not code".  What data is being locked
here?  It's the subsystem's per-cpu resources which we want to lock.  We
shouldn't consider the lock as being some way of preventing an event from
happening.

> > 
> > > 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.

Yes, we could probably fix cpufreq using the existing lock_cpu_hotplug(). 
But we have a quite large amount of racy-wrt-cpu-hotplug code in the kernel
and although a lot of it can be fixed with preempt_disable(), it's possible
that we'll get into scalability problems.

If we do have scalability problems, they can be fixed on a per-subsystem
basis: the affected subsystem can use per-cpu locking of its per-cpu data
within its CPU_UP_PREPARE and CPU_DOWN_PREPARE handlers.  That's a local,
contained issue, and addressing it this way is better than inventing (and
debugging) some fancy new lock type.

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