Re: [PATCH 05/11] smp: Enable preemption early in smp_call_function_many_cond

["Chuyi Zhou" <zhouchuyi@bytedance.com>]

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?

Before takedown_cpu(), all tasks need to be migrated to other CPUs, and 
all kthreads on that CPU must be parked, except the stopper thread and 
the hotplug thread.



> --- a/kernel/smp.c
> +++ b/kernel/smp.c
> @@ -802,19 +802,20 @@ static void smp_call_function_many_cond(
>   					unsigned int scf_flags,
>   					smp_cond_func_t cond_func)
>   {
> -	int cpu, last_cpu, this_cpu = smp_processor_id();
> -	struct call_function_data *cfd;
> +	struct call_function_data *cfd = this_cpu_ptr(&cfd_data);
> +	struct cpumask *cpumask = cfd->cpumask;
>   	bool wait = scf_flags & SCF_WAIT;
> -	bool preemptible_wait = true;
>   	cpumask_var_t cpumask_stack;
> -	struct cpumask *cpumask;
> +	int cpu, last_cpu, this_cpu;
>   	int nr_cpus = 0;
>   	bool run_remote = false;
>   
> -	lockdep_assert_preemption_disabled();
> +	if (wait && !alloc_cpumask_var(&cpumask_stack, GFP_ATOMIC))
> +		cpumask = cpumask_stack;
>   
> -	if (!alloc_cpumask_var(&cpumask_stack, GFP_ATOMIC))
> -		preemptible_wait = false;
> +	cpus_read_lock();
> +	preempt_disable();
> +	this_cpu = smp_processor_id();
>   
>   	/*
>   	 * Can deadlock when called with interrupts disabled.
> @@ -836,10 +837,6 @@ static void smp_call_function_many_cond(
>   
>   	/* Check if we need remote execution, i.e., any CPU excluding this one. */
>   	if (cpumask_any_and_but(mask, cpu_online_mask, this_cpu) < nr_cpu_ids) {
> -		cfd = this_cpu_ptr(&cfd_data);
> -
> -		cpumask = preemptible_wait ? cpumask_stack : cfd->cpumask;
> -
>   		cpumask_and(cpumask, mask, cpu_online_mask);
>   		__cpumask_clear_cpu(this_cpu, cpumask);
>   
> @@ -897,6 +894,7 @@ static void smp_call_function_many_cond(
>   		csd_do_func(func, info, NULL);
>   		local_irq_restore(flags);
>   	}
> +	preempt_enable();
>   
>   	if (run_remote && wait) {
>   		for_each_cpu(cpu, cpumask) {
> @@ -907,8 +905,8 @@ static void smp_call_function_many_cond(
>   		}
>   	}
>   
> -	if (preemptible_wait)
> -		free_cpumask_var(cpumask_stack);
> +	cpus_read_unlock();
> +	free_cpumask_var(cpumask_stack);
>   }
>   
>   /**