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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff88811f58fb78 of 8 bytes by task 14478 on cpu 0:
 shmem_add_to_page_cache+0x503/0x810 mm/shmem.c:656
 shmem_getpage_gfp+0x804/0x18f0 mm/shmem.c:1857
 shmem_fault+0x16e/0x440 mm/shmem.c:2044
 __do_fault mm/memory.c:3453 [inline]
 do_read_fault+0x41f/0x720 mm/memory.c:3846
 do_fault mm/memory.c:3974 [inline]
 handle_pte_fault mm/memory.c:4214 [inline]
 __handle_mm_fault mm/memory.c:4346 [inline]
 handle_mm_fault+0x12ba/0x18a0 mm/memory.c:4383
 faultin_page mm/gup.c:887 [inline]
 __get_user_pages+0xacb/0x1170 mm/gup.c:1113
 populate_vma_page_range mm/gup.c:1452 [inline]
 __mm_populate+0x256/0x340 mm/gup.c:1500
 mm_populate include/linux/mm.h:2605 [inline]
 vm_mmap_pgoff+0x13b/0x170 mm/util.c:511
 ksys_mmap_pgoff+0xdf/0x3a0 mm/mmap.c:1595
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811f58fb78 of 8 bytes by task 14483 on cpu 1:
 shmem_recalc_inode mm/shmem.c:305 [inline]
 shmem_getpage_gfp+0xb31/0x18f0 mm/shmem.c:1867
 shmem_fault+0x16e/0x440 mm/shmem.c:2044
 __do_fault mm/memory.c:3453 [inline]
 do_shared_fault+0xc3/0x750 mm/memory.c:3900
 do_fault mm/memory.c:3978 [inline]
 handle_pte_fault mm/memory.c:4214 [inline]
 __handle_mm_fault mm/memory.c:4346 [inline]
 handle_mm_fault+0x14d3/0x18a0 mm/memory.c:4383
 do_user_addr_fault+0x393/0x810 arch/x86/mm/fault.c:1295
 handle_page_fault arch/x86/mm/fault.c:1365 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1418
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:565
 copy_user_enhanced_fast_string+0xe/0x30 arch/x86/lib/copy_user_64.S:204
 copy_user_generic arch/x86/include/asm/uaccess_64.h:37 [inline]
 raw_copy_to_user arch/x86/include/asm/uaccess_64.h:74 [inline]
 _copy_to_user+0x7a/0xa0 lib/usercopy.c:30
 copy_to_user include/linux/uaccess.h:152 [inline]
 extract_crng_user drivers/char/random.c:1084 [inline]
 urandom_read_nowarn+0x188/0x350 drivers/char/random.c:1835
 __do_sys_getrandom drivers/char/random.c:2032 [inline]
 __se_sys_getrandom drivers/char/random.c:2007 [inline]
 __x64_sys_getrandom+0xd6/0x130 drivers/char/random.c:2007
 do_syscall_64+0x71/0x120 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 14483 Comm: syz-executor.5 Not tainted 5.8.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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