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

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

Hi Catalin,

I test this patch in your way using both EDK2 V2.6 and EDK2 v2.7. it's peculiar that this issue shows up on v2.6 but not on v2.7.
For now, I only find that if "CONFIG_DEBUG_LOCK_ALLOC" is enabled, the kernel boot will hang. However, I can't debug it by printk as this issue happens before pl11 is ready.
I will go on debugging, but very appreciated if someone can give some hints on it.

Thanks
Jianyong

> -----Original Message-----
> From: Catalin Marinas <catalin.marinas@arm.com>
> Sent: Thursday, January 6, 2022 2:04 AM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: will@kernel.org; Anshuman Khandual <Anshuman.Khandual@arm.com>;
> akpm@linux-foundation.org; david@redhat.com; quic_qiancai@quicinc.com;
> ardb@kernel.org; linux-kernel@vger.kernel.org; linux-arm-
> kernel@lists.infradead.org; gshan@redhat.com; Justin He
> <Justin.He@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create
> pud mapping
> 
> On Thu, Dec 16, 2021 at 04:28:12PM +0800, Jianyong Wu wrote:
> > The 'fixmap' is a global resource and is used recursively by create
> > pud mapping(), leading to a potential race condition in the presence
> > of a concurrent call to alloc_init_pud():
> >
> > kernel_init thread                          virtio-mem workqueue thread
> > ==================                          ===========================
> >
> >   alloc_init_pud(...)                       alloc_init_pud(...)
> >   pudp = pud_set_fixmap_offset(...)         pudp = pud_set_fixmap_offset(...)
> >   READ_ONCE(*pudp)
> >   pud_clear_fixmap(...)
> >                                             READ_ONCE(*pudp) // CRASH!
> >
> > As kernel may sleep during creating pud mapping, introduce a mutex
> > lock to serialise use of the fixmap entries by alloc_init_pud().
> >
> > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> 
> I tried to queue this patch but with certain configurations it doesn't boot
> under Qemu. Starting from defconfig, update .config with (I had this in one
> of my build scripts):
> 
> $ ./scripts/config \
> 		-e DEBUG_KERNEL \
> 		-e DEBUG_PAGEALLOC \
> 		-e DEBUG_PAGEALLOC_ENABLE_DEFAULT \
> 		-e DEBUG_WX \
> 		-e DEBUG_SET_MODULE_RONX \
> 		-e DEBUG_ALIGN_RODATA \
> 		-e ARM64_PTDUMP_DEBUGFS \
> 		-e DEBUG_OBJECTS \
> 		-e DEBUG_OBJECTS_FREE \
> 		-e DEBUG_OBJECTS_TIMERS \
> 		-e DEBUG_KOBJECT_RELEASE \
> 		-e DEBUG_LOCKING_API_SELFTESTS \
> 		-e DEBUG_PREEMPT \
> 		-e DEBUG_TIMEKEEPING \
> 		-e DEBUG_VM \
> 		-e DEBUG_VM_VMACACHE \
> 		-e DEBUG_VM_RB \
> 		-e DEBUG_VM_PGFLAGS \
> 		-e DEBUG_VIRTUAL \
> 		-e DEBUG_LIST \
> 		-e DEBUG_PI_LIST \
> 		-e DEBUG_SG \
> 		-e PROVE_LOCKING \
> 		-e DEBUG_RT_MUTEXES \
> 		-e DEBUG_ATOMIC_SLEEP \
> 		-e ATOMIC64_SELFTEST
> 
> It stop after exiting the EFI boot services. I did not have time to debug.
> 
> --
> Catalin