Re: [PATCH v3] arm64: paravirt: Use RCU read locks to guard stolen_time

["Srivatsa S. Bhat" <srivatsa@csail.mit.edu>]

On 5/13/22 10:46 AM, Elliot Berman wrote:
> From: Prakruthi Deepak Heragu <quic_pheragu@quicinc.com>
> 
> During hotplug, the stolen time data structure is unmapped and memset.
> There is a possibility of the timer IRQ being triggered before memset
> and stolen time is getting updated as part of this timer IRQ handler. This
> causes the below crash in timer handler -
> 
>   [ 3457.473139][    C5] Unable to handle kernel paging request at virtual address ffffffc03df05148
>   ...
>   [ 3458.154398][    C5] Call trace:
>   [ 3458.157648][    C5]  para_steal_clock+0x30/0x50
>   [ 3458.162319][    C5]  irqtime_account_process_tick+0x30/0x194
>   [ 3458.168148][    C5]  account_process_tick+0x3c/0x280
>   [ 3458.173274][    C5]  update_process_times+0x5c/0xf4
>   [ 3458.178311][    C5]  tick_sched_timer+0x180/0x384
>   [ 3458.183164][    C5]  __run_hrtimer+0x160/0x57c
>   [ 3458.187744][    C5]  hrtimer_interrupt+0x258/0x684
>   [ 3458.192698][    C5]  arch_timer_handler_virt+0x5c/0xa0
>   [ 3458.198002][    C5]  handle_percpu_devid_irq+0xdc/0x414
>   [ 3458.203385][    C5]  handle_domain_irq+0xa8/0x168
>   [ 3458.208241][    C5]  gic_handle_irq.34493+0x54/0x244
>   [ 3458.213359][    C5]  call_on_irq_stack+0x40/0x70
>   [ 3458.218125][    C5]  do_interrupt_handler+0x60/0x9c
>   [ 3458.223156][    C5]  el1_interrupt+0x34/0x64
>   [ 3458.227560][    C5]  el1h_64_irq_handler+0x1c/0x2c
>   [ 3458.232503][    C5]  el1h_64_irq+0x7c/0x80
>   [ 3458.236736][    C5]  free_vmap_area_noflush+0x108/0x39c
>   [ 3458.242126][    C5]  remove_vm_area+0xbc/0x118
>   [ 3458.246714][    C5]  vm_remove_mappings+0x48/0x2a4
>   [ 3458.251656][    C5]  __vunmap+0x154/0x278
>   [ 3458.255796][    C5]  stolen_time_cpu_down_prepare+0xc0/0xd8
>   [ 3458.261542][    C5]  cpuhp_invoke_callback+0x248/0xc34
>   [ 3458.266842][    C5]  cpuhp_thread_fun+0x1c4/0x248
>   [ 3458.271696][    C5]  smpboot_thread_fn+0x1b0/0x400
>   [ 3458.276638][    C5]  kthread+0x17c/0x1e0
>   [ 3458.280691][    C5]  ret_from_fork+0x10/0x20
> 
> As a fix, introduce rcu lock to update stolen time structure.
> 
> Suggested-by: Will Deacon <will@kernel.org>
> Signed-off-by: Prakruthi Deepak Heragu <quic_pheragu@quicinc.com>
> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> ---

Looks good to me, but one quick question though (see below).

Reviewed-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>

>  
>  static int stolen_time_cpu_down_prepare(unsigned int cpu)
>  {
> +	struct pvclock_vcpu_stolen_time *kaddr = NULL;
>  	struct pv_time_stolen_time_region *reg;
>  
>  	reg = this_cpu_ptr(&stolen_time_region);
>  	if (!reg->kaddr)
>  		return 0;
>  
> -	memunmap(reg->kaddr);
> -	memset(reg, 0, sizeof(*reg));
> +	kaddr = rcu_replace_pointer(reg->kaddr, NULL, true);
> +	synchronize_rcu();
> +	memunmap(kaddr);
>  

The original code used to memset the stolen time region, but this
patch seems to drop it. Was that change intentional?

Regards,
Srivatsa