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BUG: KCSAN: data-race in fat16_ent_put / fat_add_entries

write to 0xffff88811cc7a300 of 2 bytes by task 18931 on cpu 0:
 fat16_ent_put+0x54/0x90 fs/fat/fatent.c:181
 fat_alloc_clusters+0x4e1/0xbf0 fs/fat/fatent.c:501
 fat_add_cluster+0x3b/0xd0 fs/fat/inode.c:108
 fat_fallocate+0x13b/0x1d0 fs/fat/file.c:291
 vfs_fallocate+0x3b1/0x650 fs/open.c:309
 ioctl_preallocate+0x12a/0x190 fs/ioctl.c:504
 file_ioctl fs/ioctl.c:544 [inline]
 do_vfs_ioctl+0xa6b/0xd00 fs/ioctl.c:742
 ksys_ioctl+0xb2/0x150 fs/ioctl.c:761
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811cc7a300 of 1 bytes by task 18936 on cpu 1:
 fat_add_entries+0x148/0xb70 fs/fat/dir.c:1307
 vfat_add_entry+0x1750/0x1b50 fs/fat/namei_vfat.c:677
 vfat_mkdir+0xbb/0x310 fs/fat/namei_vfat.c:865
 vfs_mkdir+0x281/0x390 fs/namei.c:3889
 do_mkdirat+0x1b5/0x200 fs/namei.c:3912
 __do_sys_mkdir fs/namei.c:3928 [inline]
 __se_sys_mkdir fs/namei.c:3926 [inline]
 __x64_sys_mkdir+0x3d/0x50 fs/namei.c:3926
 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: 18936 Comm: syz-executor.5 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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