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

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Message-Id: <87h8z8g6wu.fsf@linux.vnet.ibm.com>
Date:   Wed, 21 Jun 2017 22:14:41 -0300
From:   Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        John Allen <jallen@...ux.vnet.ibm.com>,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>,
        Nathan Fontenot <nfont@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] powerpc: Only obtain cpu_hotplug_lock if called by rtasd


Michael Ellerman <mpe@...erman.id.au> writes:
> 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?

It's been broken since at least v4.4, I think. I don't know about
earlier versions.

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

It's not hard to backport both this patch and commit fe5595c07400
("stop_machine: Provide stop_machine_cpuslocked()") from branch
smp/hotplug in tip.git for stable.

Since rtasd only started calling arch_update_cpu_topology since v4.11,
for earlier versions this patch can be simplified to making that
function call stop_machine_cpuslocked unconditionally instead of
defining a separate function.

>> 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?

Yes. I said "probably" because I don't know if you want to wait
until that branch is merged so that you can carry this patch in your
tree.

-- 
Thiago Jung Bauermann
IBM Linux Technology Center