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BUG: KCSAN: data-race in memchr / string

write to 0xffffffff8453890b of 1 bytes by task 13468 on cpu 1:
 string_nocheck lib/vsprintf.c:641 [inline]
 string+0xa1/0x210 lib/vsprintf.c:719
 vsnprintf+0xab0/0xed0 lib/vsprintf.c:2790
 vscnprintf+0x29/0x80 lib/vsprintf.c:2893
 printk_sprint kernel/printk/printk.c:2012 [inline]
 vprintk_store+0x4c1/0x9c0 kernel/printk/printk.c:2115
 vprintk_emit+0xca/0x3d0 kernel/printk/printk.c:2157
 vprintk_default+0x22/0x30 kernel/printk/printk.c:2185
 vprintk+0x15a/0x170 kernel/printk/printk_safe.c:392
 printk+0x62/0x87 kernel/printk/printk.c:2216
 p9_virtio_create+0xf5/0x180 net/9p/trans_virtio.c:680
 p9_client_create+0x56a/0x6f0 net/9p/client.c:1041
 v9fs_session_init+0x100/0xcf0 fs/9p/v9fs.c:406
 v9fs_mount+0x57/0x4c0 fs/9p/vfs_super.c:126
 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_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff8453890b of 1 bytes by task 13451 on cpu 0:
 memchr+0x34/0x50 lib/string.c:1054
 count_lines kernel/printk/printk_ringbuffer.c:1675 [inline]
 copy_data kernel/printk/printk_ringbuffer.c:1721 [inline]
 prb_read kernel/printk/printk_ringbuffer.c:1814 [inline]
 _prb_read_valid+0x5bf/0x8d0 kernel/printk/printk_ringbuffer.c:1880
 prb_read_valid_info+0x57/0x80 kernel/printk/printk_ringbuffer.c:1963
 find_first_fitting_seq+0x64/0x2e0 kernel/printk/printk.c:1456
 syslog_print_all kernel/printk/printk.c:1574 [inline]
 do_syslog+0x6be/0x1140 kernel/printk/printk.c:1675
 __do_sys_syslog kernel/printk/printk.c:1756 [inline]
 __se_sys_syslog kernel/printk/printk.c:1754 [inline]
 __x64_sys_syslog+0x3f/0x50 kernel/printk/printk.c:1754
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x5b -> 0x0a

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 13451 Comm: syz-executor.4 Tainted: G        W         5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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