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SubjectRe: [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

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