Re: [RFC PATCH v2 01/10] CPU hotplug: Provide APIs for "light"atomic readers to prevent CPU offline

[Steven Rostedt]

On Fri, 2012-12-07 at 01:06 +0530, Srivatsa S. Bhat wrote:
> On 12/07/2012 12:58 AM, Steven Rostedt wrote:
> > On Fri, 2012-12-07 at 00:18 +0530, Srivatsa S. Bhat wrote:
> >> On 12/06/2012 09:48 PM, Oleg Nesterov wrote:
> >>> On 12/06, Srivatsa S. Bhat wrote:
> >>>>
> >>>> +void get_online_cpus_atomic(void)
> >>>> +{
> >>>> +	int c, old;
> >>>> +
> >>>> +	preempt_disable();
> >>>> +	read_lock(&hotplug_rwlock);
> >>>
> >>> Confused... Why it also takes hotplug_rwlock?
> >>
> >> To avoid ABBA deadlocks.
> >>
> >> hotplug_rwlock was meant for the "light" readers.
> >> The atomic counters were meant for the "heavy/full" readers.
> >> I wanted them to be able to nest in any manner they wanted,
> >> such as:
> >>
> >> Full inside light:
> >>
> >> get_online_cpus_atomic_light()
> >> 	...
> >> 	get_online_cpus_atomic_full()
> >> 	...
> >> 	put_online_cpus_atomic_full()
> >> 	...
> >> put_online_cpus_atomic_light()
> >>
> >> Or, light inside full:
> >>
> >> get_online_cpus_atomic_full()
> >> 	...
> >> 	get_online_cpus_atomic_light()
> >> 	...
> >> 	put_online_cpus_atomic_light()
> >> 	...
> >> put_online_cpus_atomic_full()
> >>
> >> 



> The root-cause of this deadlock is again lock-ordering mismatch right?
> CPU0 takes locks in order A, B
> CPU1 takes locks in order B, A
> 
> And the writer facilitates in actually getting deadlocked.
> 
> I avoid this in this patchset by always taking the locks in the same
> order. So we won't be deadlocking like this.

OK, I haven't looked closely at the patch yet. I'm currently hacking on
my own problems. But just from the description above, it looked like you
were using rw_locks() to be able to inverse the order of the locks.

-- Steve


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