linux-kernel - Understanding stop_machine() use for cpu

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [day] [month] [year] [list]

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