Re: [PATCH] Relay CPU Hotplug support

On Fri, Dec 22, 2006 at 02:44:58AM -0800, Andrew Morton wrote:
> On Fri, 22 Dec 2006 16:07:24 +0530
> Gautham R Shenoy <[email protected]> wrote:
> 
> > While we are at this per-subsystem cpuhotplug "locking", here's a
> > proposal that might put an end to the workqueue deadlock woes.
> 
> Oleg is working on some patches which will permit us to cancel or wait upon
> a particular work_struct, rather than upon all pending work_structs.
>

Oh! I was refering to the other set of workqueue deadlock woes. The
ones caused when subsystems (like cpufreq) try to create/destroy
workqueue from the cpuhotplug callback path. 

Creation/destruction of workqueue requires us to take workqueue_mutex,
which would have already been taken during CPU_LOCK_ACQUIRE.

More often than not, the cpu hotplug protection that we need
is while accessing either cpu_online_map OR one of it's persubsystem
mirrors like policy->cpus. 
So it makes more sense to have all the persubsystem 
mutexes held only during the cpu-hotplug operation (i.e stop_machine_run
and __cpu_up) and release them immediately after sending notifications to
update the persubsystem online_cpu map.

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"