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BUG: KCSAN: data-race in copy_page_to_iter / string

write to 0xffff888135ce9050 of 1 bytes by task 7740 on cpu 1:
 string_nocheck lib/vsprintf.c:615 [inline]
 string+0xa1/0x210 lib/vsprintf.c:693
 vsnprintf+0xab0/0xed0 lib/vsprintf.c:2752
 snprintf+0x6f/0x90 lib/vsprintf.c:2885
 disk_name block/genhd.c:96 [inline]
 bdevname+0xaf/0xe0 block/genhd.c:107
 ext4_multi_mount_protect+0x521/0x6a0 fs/ext4/mmp.c:378
 ext4_fill_super+0x49bd/0x8010 fs/ext4/super.c:4780
 mount_bdev+0x1e8/0x290 fs/super.c:1368
 ext4_mount+0x2d/0x40 fs/ext4/super.c:6632
 legacy_get_tree+0x70/0xc0 fs/fs_context.c:592
 vfs_get_tree+0x4a/0x1a0 fs/super.c:1498
 do_new_mount fs/namespace.c:2905 [inline]
 path_mount+0x11d4/0x1ce0 fs/namespace.c:3235
 do_mount fs/namespace.c:3248 [inline]
 __do_sys_mount fs/namespace.c:3456 [inline]
 __se_sys_mount+0x23d/0x2e0 fs/namespace.c:3433
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3433
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888135ce9000 of 1024 bytes by task 7735 on cpu 0:
 instrument_copy_to_user include/linux/instrumented.h:119 [inline]
 copyout lib/iov_iter.c:189 [inline]
 copy_page_to_iter_iovec lib/iov_iter.c:261 [inline]
 copy_page_to_iter+0x320/0x860 lib/iov_iter.c:973
 filemap_read+0xf06/0x1230 mm/filemap.c:2605
 generic_file_read_iter+0x75/0x2c0 mm/filemap.c:2701
 blkdev_read_iter+0xb9/0xf0 fs/block_dev.c:1732
 call_read_iter include/linux/fs.h:2108 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x565/0x5d0 fs/read_write.c:496
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7735 Comm: systemd-udevd Not tainted 5.13.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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