On Tue, May 29, 2007 at 01:56:24PM -0700, Linus Torvalds wrote:
> As far as I'm concerned, we should
> - use "preempt_disable()" to protect against CPU's coming and going
> - use "stop_machine()" or similar that already honors preemption, and
> which I trust a whole lot more than freezer.
> - .. especially since this is already how we are supposed to be protected
> against CPU's going away, and we've already started doing that (for an
> example of this, see things like e18f3ffb9c from Andrew)
>
> It really does seem fairly straightforward to make "__cpu_up()" be called
> through stop_machine too. Looking at _cpu_down:
>
> mutex_lock(&cpu_bitmask_lock);
> p = __stop_machine_run(take_cpu_down, NULL, cpu);
> mutex_unlock(&cpu_bitmask_lock);
>
> and then looking at _cpu_up:
>
> mutex_lock(&cpu_bitmask_lock);
> ret = __cpu_up(cpu);
> mutex_unlock(&cpu_bitmask_lock);
>
> I just go "Aww, wouldn't it be nice to just make that "__cpu_up()" call be
> done through __stop_machine_run() too?"
>
> Hmm?
>
> Then, you could get the "cpu_bitmask_lock" if you need to sleep,
and that's where all the problems started - sleepers needing to take that mutex
recursively (which we did/do not support).
foo() takes cpu_bitmask_lock and calls
foo_bar() which also needs cpu_bitmask_lock
What is a solution to that?
- Forget (hide?) this whole locking mess by using freezer, which
is what Andrew wanted us to shoot for :) I am somewhat biased
with Andrew here in that I think it will lead to more stabler cpu
hotplug code over time. Again I know some people will beg to differ
on this view.
- extend mutexes to support recursion (which I gather Linux has
religiously avoided so far)
- invent a special lock for cpu hotplug which supports recursion.
This is what Gautham tried doing with [1], with the bonus that it
made the lock extremely scalable for readers by using per-cpu
reference counters and RCU. He is preparing to resend those patches
against latest kernel atm
- Anything else you can think of?
[1] http://lkml.org/lkml/2006/10/26/73
> but if you don't want to do that (and quite often you don't), just doing a
> "preempt_disable()" or taking a spinlock will *also* guarantee that no new
> CPU's suddenly show up, so it's safe to look at the CPU online bitmasks.
>
> Do we really need anything else?
see above
> As mentioned, it's actually fairly easy to add verification calls to make
> sure that certain accesses are done with preemption disabled, so..
--
Regards,
vatsa
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