linux-kernel - Re: [PATCH] powerpc: Only obtain cpu_hotplug

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Message-ID: <8737attzw5.fsf@concordia.ellerman.id.au>
Date:   Wed, 21 Jun 2017 20:10:18 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        linuxppc-dev@...ts.ozlabs.org
Cc:     linux-kernel@...r.kernel.org,
        John Allen <jallen@...ux.vnet.ibm.com>,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] powerpc: Only obtain cpu_hotplug_lock if called by rtasd

Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com> writes:

> Calling arch_update_cpu_topology from a CPU hotplug state machine callback
> hits a deadlock because the function tries to get a read lock on
> cpu_hotplug_lock while the state machine still holds a write lock on it.
>
> Since all callers of arch_update_cpu_topology except rtasd already hold
> cpu_hotplug_lock, this patch changes the function to use
> stop_machine_cpuslocked and creates a separate function for rtasd which
> still tries to obtain the lock.
>
> Michael Bringmann investigated the bug and provided a detailed analysis
> of the deadlock on this previous RFC for an alternate solution:
>
> https://patchwork.ozlabs.org/patch/771293/

Do we know when this broke? Or has it never worked?

Should it go to stable? (can't in its current form AFAICS)

> Signed-off-by: Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
> ---
>
> Notes:
>     This patch applies on tip/smp/hotplug, it should probably be carried there.

stop_machine_cpuslocked() doesn't exist in mainline so I think it has to
be carried there right?

cheers