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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_getpage_gfp

write to 0xffff88808eec6348 of 8 bytes by task 29486 on cpu 1:
 shmem_add_to_page_cache+0x5d7/0x900 mm/shmem.c:718
 shmem_getpage_gfp+0x821/0x19c0 mm/shmem.c:1920
 shmem_fault+0x16c/0x430 mm/shmem.c:2107
 __do_fault mm/memory.c:3639 [inline]
 do_shared_fault+0xc3/0x7c0 mm/memory.c:4086
 do_fault mm/memory.c:4164 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x1710/0x1af0 mm/memory.c:4633
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538
 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:168 [inline]
 extract_crng_user drivers/char/random.c:1084 [inline]
 urandom_read_nowarn+0x1a9/0x370 drivers/char/random.c:1836
 __do_sys_getrandom drivers/char/random.c:2033 [inline]
 __se_sys_getrandom drivers/char/random.c:2008 [inline]
 __x64_sys_getrandom+0xd6/0x130 drivers/char/random.c:2008
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88808eec6348 of 8 bytes by task 29471 on cpu 0:
 shmem_recalc_inode mm/shmem.c:367 [inline]
 shmem_getpage_gfp+0xb7c/0x19c0 mm/shmem.c:1930
 shmem_fault+0x16c/0x430 mm/shmem.c:2107
 __do_fault mm/memory.c:3639 [inline]
 do_read_fault+0x41f/0x760 mm/memory.c:4032
 do_fault mm/memory.c:4160 [inline]
 handle_pte_fault mm/memory.c:4400 [inline]
 __handle_mm_fault mm/memory.c:4535 [inline]
 handle_mm_fault+0x14e9/0x1af0 mm/memory.c:4633
 faultin_page mm/gup.c:878 [inline]
 __get_user_pages+0xa3d/0x1010 mm/gup.c:1090
 populate_vma_page_range mm/gup.c:1423 [inline]
 __mm_populate+0x24d/0x340 mm/gup.c:1471
 mm_populate include/linux/mm.h:2566 [inline]
 vm_mmap_pgoff+0x133/0x170 mm/util.c:511
 ksys_mmap_pgoff+0xe1/0x380 mm/mmap.c:1596
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 29471 Comm: syz-executor.3 Not tainted 5.9.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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