Understanding stop_machine() use for cpu_down()

From: Hardik H Bagdi <hbagdi1@binghamton.edu>
To: linux-hotplug@vger.kernel.org
Subject: Understanding stop_machine() use for cpu_down()
Date: Wed, 25 Jan 2017 23:41:53 +0000	[thread overview]
Message-ID: <CADdiZBT-wvAmmtRem1=arnvPcyJrgH3s43ngAuzqk3poHe-FJA@mail.gmail.com> (raw)

Hi,

I'm trying to understand why stop_machine() is necessary for
cpu_down() operation.

I see that multi_cpu_stop() on every online cpu (which hogs the cpu
and then triggers state changes state)
and then, take_cpu_down is invoked on the outgoing cpu.
This happens by every cpu decrementing the msdata->thread_ack and last
one trigger a state change.

I'm trying to understand why is this necessary.

Is to wait for RCU grace period(s)?
If yes, what variable/struct is being protected and how does
stop_machine() help there?

I see there was a patch (https://lwn.net/Articles/538819/) but had
some issues with idle_loop.

Is there an alternative where the stop_machine would not be necessary?
Or more specifically, is there a reason why every CPU would need to stop?

Any help would be appreciated.

Thanks,
Hardik