linux-kernel - Re: [PATCH 05/11] smp: Enable preemption early in smp_call_function_many

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Message-ID: <20260205145912.GM2995752@noisy.programming.kicks-ass.net>
Date: Thu, 5 Feb 2026 15:59:12 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Chuyi Zhou <zhouchuyi@...edance.com>
Cc: tglx@...utronix.de, mingo@...hat.com, luto@...nel.org,
	paulmck@...nel.org, muchun.song@...ux.dev, bp@...en8.de,
	dave.hansen@...ux.intel.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 05/11] smp: Enable preemption early in
 smp_call_function_many_cond

On Thu, Feb 05, 2026 at 10:29:51PM +0800, Chuyi Zhou wrote:
> Hi Peter,
> 
> 在 2026/2/5 18:57, Peter Zijlstra 写道:
> > On Thu, Feb 05, 2026 at 10:52:36AM +0100, Peter Zijlstra wrote:
> >> On Tue, Feb 03, 2026 at 07:23:55PM +0800, Chuyi Zhou wrote:
> >>
> >>> +	/*
> >>> +	 * Prevent the current CPU from going offline.
> >>> +	 * Being migrated to another CPU and calling csd_lock_wait() may cause
> >>> +	 * UAF due to smpcfd_dead_cpu() during the current CPU offline process.
> >>> +	 */
> >>> +	migrate_disable();
> >>
> >> This is horrible crap. migrate_disable() is *NOT* supposed to be used to
> >> serialize cpu hotplug.
> > 
> > This was too complicated or something?
> > 
> 
> Now most callers of smp_call*() explicitly use preempt_disable(). IIUC, 
> if we want to use cpus_read_lock(), we first need to clean up all these 
> preempt_disable() calls.
> 
> Maybe a stupid question: Why can't migrate_disable prevent CPU removal?

It can, but migrate_disable() is horrible, it should not be used if at
all possible.