Re: [PATCH 1/2] x86/cpu/topology: don't write to immutable cpu_present_mask

From: Peter Zijlstra <peterz@infradead.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	Alexey Dobriyan <adobriyan@gmail.com>,
	mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
	x86@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org,
	Jonathan.Cameron@huawei.com, linuxarm@huawei.com,
	linux@roeck-us.net
Subject: Re: [PATCH 1/2] x86/cpu/topology: don't write to immutable cpu_present_mask
Date: Tue, 9 Apr 2024 11:24:15 +0200	[thread overview]
Message-ID: <20240409092415.GB2665@noisy.programming.kicks-ass.net> (raw)
In-Reply-To: <ZhT/ZTdRN+Gxfe8S@gmail.com>

On Tue, Apr 09, 2024 at 10:42:13AM +0200, Ingo Molnar wrote:
> 
> * Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > On Sun, Apr 07 2024 at 18:26, Alexey Dobriyan wrote:
> > > Workaround the following oops:
> > >
> > > 	topology_hotplug_apic
> > > 	topo_set_cpuids
> > > 		set_cpu_possible(cpu, true);
> > > 		// write to __ro_after_init section after init
> > 
> > Duh, yes.
> > 
> > > adobriyan: I'm not sure what's going on, can it set unset bit here?
> > > If not, then why does it repeat the job and set already set bits.
> > >
> > > Anyhow, let's not oops peoples' machines for now.
> > 
> > Adding a bandaid to paper over the non-understood real problem is
> > definitely not a good plan. I take this patch as a bug report.
> > 
> > Proper fix below.
> 
> BTW., independently of the fix, warning about a too late set_cpu_possible() 
> might still make sense - clearly it *can* be called too late by 
> architecture init code :-)

Make the function __init ? Then it goes away right when the data becomes
RO.