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Date: Fri, 10 Nov 2017 14:43:31 +0100
From: Greg KH <greg@kroah.com>
To: James Hogan <james.hogan@mips.com>
Cc: stable@vger.kernel.org, Ralf Baechle <ralf@linux-mips.org>,
        linux-mips@linux-mips.org,
        Matt Redfearn <matt.redfearn@imgtec.com>,
        Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>,
        James Hogan <jhogan@kernel.org>
Subject: Re: [PATCH BACKPORT 4.1..4.9] MIPS: SMP: Fix deadlock & online race
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References: <20171106211514.29104-1-james.hogan@mips.com>
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On Mon, Nov 06, 2017 at 09:15:14PM +0000, James Hogan wrote:
> From: Matt Redfearn <matt.redfearn@imgtec.com>
> 
> commit 9e8c399a88f0b87e41a894911475ed2a8f8dff9e upstream.
> 
> Commit 6f542ebeaee0 ("MIPS: Fix race on setting and getting
> cpu_online_mask") effectively reverted commit 8f46cca1e6c06 ("MIPS: SMP:
> Fix possibility of deadlock when bringing CPUs online") and thus has
> reinstated the possibility of deadlock.
> 
> The commit was based on testing of kernel v4.4, where the CPU hotplug
> core code issued a BUG() if the starting CPU is not marked online when
> the boot CPU returns from __cpu_up. The commit fixes this race (in
> v4.4), but re-introduces the deadlock situation.
> 
> As noted in the commit message, upstream differs in this area. Commit
> 8df3e07e7f21f ("cpu/hotplug: Let upcoming cpu bring itself fully up")
> adds a completion event in the CPU hotplug core code, making this race
> impossible. However, people were unhappy with relying on the core code
> to do the right thing.
> 
> To address the issues both commits were trying to fix, add a second
> completion event in the MIPS smp hotplug path. It removes the
> possibility of a race, since the MIPS smp hotplug code now synchronises
> both the boot and secondary CPUs before they return to the hotplug core
> code. It also addresses the deadlock by ensuring that the secondary CPU
> is not marked online before it's counters are synchronised.
> 
> This fix should also be backported to fix the race condition introduced
> by the backport of commit 8f46cca1e6c06 ("MIPS: SMP: Fix possibility of
> deadlock when bringing CPUs online"), through really that race only
> existed before commit 8df3e07e7f21f ("cpu/hotplug: Let upcoming cpu
> bring itself fully up").
> 
> Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
> Fixes: 6f542ebeaee0 ("MIPS: Fix race on setting and getting cpu_online_mask")
> CC: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>
> Cc: <stable@vger.kernel.org> # v4.1+: 8f46cca1e6c0: "MIPS: SMP: Fix possibility of deadlock when bringing CPUs online"
> Cc: <stable@vger.kernel.org> # v4.1+: a00eeede507c: "MIPS: SMP: Use a completion event to signal CPU up"
> Cc: <stable@vger.kernel.org> # v4.1+: 6f542ebeaee0: "MIPS: Fix race on setting and getting cpu_online_mask"

These did not apply to 3.18, so this patch overall did not apply there
either.

I don't know if you care about 3.18, but if so, can you provide
backports of these for that tree, and then resend this patch so I can
queue it up?

thanks,

greg k-h