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BUG: KCSAN: data-race in shmem_getpage_gfp / shmem_write_end

write to 0xffff8880a6a9ee98 of 8 bytes by task 9096 on cpu 1:
 i_size_write include/linux/fs.h:888 [inline]
 shmem_write_end+0x83/0x4f0 mm/shmem.c:2493
 generic_perform_write+0x1d3/0x320 mm/filemap.c:3315
 __generic_file_write_iter+0x251/0x380 mm/filemap.c:3433
 generic_file_write_iter+0x28c/0x390 mm/filemap.c:3465
 call_write_iter include/linux/fs.h:1895 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 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+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8880a6a9ee98 of 8 bytes by task 9099 on cpu 0:
 shmem_getpage_gfp+0x81b/0x13c0 mm/shmem.c:1763
 shmem_getpage mm/shmem.c:154 [inline]
 shmem_file_read_iter+0x224/0x720 mm/shmem.c:2559
 call_read_iter include/linux/fs.h:1889 [inline]
 generic_file_splice_read+0x35c/0x500 fs/splice.c:307
 do_splice_to+0xf2/0x130 fs/splice.c:877
 splice_direct_to_actor+0x1a1/0x510 fs/splice.c:954
 do_splice_direct+0x161/0x1e0 fs/splice.c:1063
 do_sendfile+0x384/0x7f0 fs/read_write.c:1464
 __do_sys_sendfile64 fs/read_write.c:1525 [inline]
 __se_sys_sendfile64 fs/read_write.c:1511 [inline]
 __x64_sys_sendfile64+0x12a/0x140 fs/read_write.c:1511
 do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9099 Comm: syz-executor.2 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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