RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping

[Jianyong Wu <Jianyong.Wu@arm.com>]

Hi Catalin,

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@arm.com>
> Sent: Thursday, January 27, 2022 8:34 PM
> To: David Hildenbrand <david@redhat.com>
> Cc: Jianyong Wu <Jianyong.Wu@arm.com>; Ard Biesheuvel
> <ardb@kernel.org>; Justin He <Justin.He@arm.com>; will@kernel.org;
> Anshuman Khandual <Anshuman.Khandual@arm.com>; akpm@linux-
> foundation.org; quic_qiancai@quicinc.com; linux-kernel@vger.kernel.org;
> linux-arm-kernel@lists.infradead.org; gshan@redhat.com; nd <nd@arm.com>
> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create
> pud mapping
> 
> On Thu, Jan 27, 2022 at 01:22:47PM +0100, David Hildenbrand wrote:
> > > Yes, system_state can roughly separate these callers of
> __create_pgd_mapping. When system_state > SYSTEM_BOOTING we can
> add the lock.
> > > Thus, I have the following change:
> > >
> > > static DEFINE_SPINLOCK(swapper_pgdir_lock);
> > > +static DEFINE_MUTEX(fixmap_lock);
> > >
> > >  void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)  { @@ -329,6 +330,8
> @@
> > > static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long
> end,
> > >         }
> > >         BUG_ON(p4d_bad(p4d));
> > >
> > > +       if (system_state > SYSTEM_BOOTING)
> >
> > As there is nothing smaller than SYSTEM_BOOTING, you can use
> > 	if (system_state != SYSTEM_BOOTING)
> >
> > ...
> >
> > >
> > > It seems work and somehow simper. But I don't know if it is
> > > reasonable to do this. So, any idea? @Ard Biesheuvel  @Catalin
> > > Marinas
> >
> > It's worth looking at kernel/notifier.c, e.g.,
> > blocking_notifier_chain_register()
> >
> > if (unlikely(system_state == SYSTEM_BOOTING))
> > 	return notifier_chain_register(&nh->head, n);
> >
> > down_write(&nh->rwsem);
> > ret = notifier_chain_register(&nh->head, n); up_write(&nh->rwsem);
> >
> > If we decide to go down that path, we should make sure to add a
> > comment like
> >
> > /*
> >  * No need for locking during early boot. And it doesn't work as
> >  * expected with KASLR enabled where we might clear BSS twice.
> >  */
> 
> A similar approach sounds fine to me.
> 

OK, I'll send the next version based on David's comments.

Thanks
Jianyong

> --
> Catalin