linux-kernel - Understanding stop_machine() use for cpu

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Message-ID: <CADdiZBRA3o1nW9ZT80fa2uXFwe+SB_2b=3QtankSR0695Z4tQQ@mail.gmail.com>
Date:   Thu, 26 Jan 2017 09:12:34 -0500
From:   Hardik H Bagdi <hbagdi1@...ghamton.edu>
To:     linux-kernel@...r.kernel.org
Cc:     rusty@...tcorp.com.au, vatsa@...ibm.com, ashok.raj@...el.com,
        zwanem@...il.com
Subject: Understanding stop_machine() use for cpu_down()

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