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

[Will Deacon]

On Fri, May 13, 2022 at 04:32:53PM -0700, Srivatsa S. Bhat wrote:
> On 5/13/22 10:46 AM, Elliot Berman wrote:
> > From: Prakruthi Deepak Heragu <quic_pheragu@xxxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > Signed-off-by: Prakruthi Deepak Heragu <quic_pheragu@xxxxxxxxxxx>
> > Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
> > ---
> 
> Looks good to me, but one quick question though (see below).
> 
> Reviewed-by: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>

Cheers.

> >  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?

'struct pv_time_stolen_time_region' only has one field ('kaddr'), which
we're now clearing with rcu_replace_pointer() so the memset doesn't make
sense.

Will