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BUG: KCSAN: data-race in shmem_add_to_page_cache / shmem_recalc_inode

write to 0xffff888099ef5e50 of 8 bytes by task 9820 on cpu 0:
 shmem_add_to_page_cache+0x4b0/0x6c0 mm/shmem.c:643
 shmem_getpage_gfp+0x43f/0x14c0 mm/shmem.c:1876
 shmem_fault+0x117/0x480 mm/shmem.c:2067
 __do_fault+0xa8/0x1e0 mm/memory.c:3276
 do_shared_fault mm/memory.c:3728 [inline]
 do_fault mm/memory.c:3806 [inline]
 handle_pte_fault mm/memory.c:4033 [inline]
 __handle_mm_fault+0x1d2d/0x2cf0 mm/memory.c:4163
 handle_mm_fault+0x21c/0x540 mm/memory.c:4200
 do_user_addr_fault arch/x86/mm/fault.c:1442 [inline]
 do_page_fault+0x4a4/0xa52 arch/x86/mm/fault.c:1517
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203
 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:128 [inline]
 copyout+0xa8/0xb0 lib/iov_iter.c:143
 _copy_to_iter+0x174/0x990 lib/iov_iter.c:628
 copy_to_iter include/linux/uio.h:138 [inline]
 simple_copy_to_iter+0x56/0x80 net/core/datagram.c:522
 __skb_datagram_iter+0x23f/0x470 net/core/datagram.c:452
 skb_copy_datagram_iter+0x65/0x160 net/core/datagram.c:536
 skb_copy_datagram_msg include/linux/skbuff.h:3513 [inline]
 unix_stream_read_actor+0x4b/0x80 net/unix/af_unix.c:2459
 unix_stream_read_generic+0x5b9/0x10f0 net/unix/af_unix.c:2376
 unix_stream_recvmsg+0xb8/0xf0 net/unix/af_unix.c:2475
 sock_recvmsg_nosec net/socket.c:886 [inline]
 sock_recvmsg net/socket.c:904 [inline]
 sock_recvmsg+0x8e/0xb0 net/socket.c:900
 __sys_recvfrom+0x176/0x270 net/socket.c:2055
 __do_sys_recvfrom net/socket.c:2073 [inline]
 __se_sys_recvfrom net/socket.c:2069 [inline]
 __x64_sys_recvfrom+0x7e/0xa0 net/socket.c:2069
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888099ef5e50 of 8 bytes by task 9773 on cpu 1:
 shmem_recalc_inode+0x50/0x180 mm/shmem.c:306
 shmem_getpage_gfp+0x5ca/0x14c0 mm/shmem.c:1890
 shmem_fault+0x117/0x480 mm/shmem.c:2067
 __do_fault+0xa8/0x1e0 mm/memory.c:3276
 do_read_fault mm/memory.c:3673 [inline]
 do_fault mm/memory.c:3802 [inline]
 handle_pte_fault mm/memory.c:4033 [inline]
 __handle_mm_fault+0x2611/0x2cf0 mm/memory.c:4163
 handle_mm_fault+0x21c/0x540 mm/memory.c:4200
 do_user_addr_fault arch/x86/mm/fault.c:1442 [inline]
 do_page_fault+0x4a4/0xa52 arch/x86/mm/fault.c:1517
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203
 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_from_user arch/x86/include/asm/uaccess_64.h:71 [inline]
 copyin+0xab/0xb0 lib/iov_iter.c:152
 copy_page_from_iter_iovec lib/iov_iter.c:296 [inline]
 copy_page_from_iter+0x250/0x600 lib/iov_iter.c:942
 skb_copy_datagram_from_iter+0x262/0x3e0 net/core/datagram.c:582
 unix_stream_sendmsg+0x483/0x850 net/unix/af_unix.c:1895
 sock_sendmsg_nosec net/socket.c:652 [inline]
 sock_sendmsg+0x98/0xc0 net/socket.c:672
 sock_write_iter+0x186/0x240 net/socket.c:1004
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0x16a/0x1a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9773 Comm: syz-executor.4 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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