Messages in this thread Patch in this message | | | Date | Wed, 15 Dec 2021 09:13:37 -0500 | From | Qian Cai <> | Subject | Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping |
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On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote: > fixmap is a global resource and is used recursively in create pud mapping. > It may lead to race condition when alloc_init_pud is called concurrently. > > Fox example: > alloc_init_pud is called when kernel_init. If memory hotplug > thread, which will also call alloc_init_pud, happens during > kernel_init, the race for fixmap occurs. > > The race condition flow can be: > > *************** begin ************** > > kerenl_init thread virtio-mem workqueue thread > ================== ======== ================== > alloc_init_pud(...) > pudp = pud_set_fixmap_offset(..) alloc_init_pud(...) > ... ... > READ_ONCE(*pudp) //OK! pudp = pud_set_fixmap_offset( > ... ... > pud_clear_fixmap() //fixmap break > READ_ONCE(*pudp) //CRASH! > > **************** end *************** > > Hence, a spin lock is introduced to protect the fixmap during create pdg > mapping. > > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
I am afraid there is a problem to take a spinlock there.
node 0 deferred pages initialised in 2740ms pgdatinit0 (176) used greatest stack depth: 59184 bytes left devtmpfs: initialized KASLR disabled due to lack of seed BUG: sleeping function called from invalid context at mm/page_alloc.c:5151 in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: swapper/0 preempt_count: 1, expected: 0 1 lock held by swapper/0/1: #0: ffff800009ea3278 (fixmap_lock){+.+.}-{2:2}, at: __create_pgd_mapping alloc_init_pud at /usr/src/linux-next/arch/arm64/mm/mmu.c:340 (discriminator 4) (inlined by) __create_pgd_mapping at /usr/src/linux-next/arch/arm64/mm/mmu.c:393 (discriminator 4) CPU: 0 PID: 1 Comm: swapper/0 Tainted: G W 5.16.0-rc5-next-20211214 Call trace: dump_backtrace show_stack dump_stack_lvl dump_stack __might_resched __might_sleep __alloc_pages alloc_page_interleave alloc_pages __get_free_pages __pgd_pgtable_alloc __create_pgd_mapping __phys_to_pte_val at /usr/src/linux-next/./arch/arm64/include/asm/pgtable.h:77 (inlined by) __pud_populate at /usr/src/linux-next/./arch/arm64/include/asm/pgalloc.h:25 (inlined by) alloc_init_cont_pmd at /usr/src/linux-next/arch/arm64/mm/mmu.c:277 (inlined by) alloc_init_pud at /usr/src/linux-next/arch/arm64/mm/mmu.c:358 (inlined by) __create_pgd_mapping at /usr/src/linux-next/arch/arm64/mm/mmu.c:393 map_entry_trampoline map_entry_trampoline at /usr/src/linux-next/arch/arm64/mm/mmu.c:639 do_one_initcall kernel_init_freeable kernel_init ret_from_fork
> --- > arch/arm64/mm/mmu.c | 7 +++++++ > 1 file changed, 7 insertions(+) > > diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c > index acfae9b41cc8..98ac09ae9588 100644 > --- a/arch/arm64/mm/mmu.c > +++ b/arch/arm64/mm/mmu.c > @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused; > static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused; > > static DEFINE_SPINLOCK(swapper_pgdir_lock); > +static DEFINE_SPINLOCK(fixmap_lock); > > void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd) > { > @@ -329,6 +330,11 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > } > BUG_ON(p4d_bad(p4d)); > > + /* > + * fixmap is global resource, thus it needs to be protected by a lock > + * in case of race condition. > + */ > + spin_lock(&fixmap_lock); > pudp = pud_set_fixmap_offset(p4dp, addr); > do { > pud_t old_pud = READ_ONCE(*pudp); > @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end, > } while (pudp++, addr = next, addr != end); > > pud_clear_fixmap(); > + spin_unlock(&fixmap_lock); > } > > static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys, > -- > 2.17.1 >
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