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BUG: KCSAN: data-race in memchr / vsnprintf

write to 0xffffffff88e27527 of 32 bytes by task 31363 on cpu 0:
 vsnprintf+0x2ce/0x890 lib/vsprintf.c:2878
 vscnprintf+0x41/0x90 lib/vsprintf.c:2991
 printk_sprint+0x30/0x2d0 kernel/printk/printk.c:2189
 vprintk_store+0x599/0x860 kernel/printk/printk.c:2309
 vprintk_emit+0x10d/0x580 kernel/printk/printk.c:2399
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2438
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2448
 set_capacity_and_notify+0x14c/0x1f0 block/genhd.c:93
 loop_set_size+0x2e/0x70 drivers/block/loop.c:220
 loop_configure+0x825/0x9c0 drivers/block/loop.c:1077
 lo_ioctl+0x1e1/0x12b0 drivers/block/loop.c:1532
 blkdev_ioctl+0x356/0x440 block/ioctl.c:705
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x1816/0x3000 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff88e27537 of 1 bytes by task 31361 on cpu 1:
 memchr+0x28/0x50 lib/string.c:809
 memchr include/linux/fortify-string.h:739 [inline]
 count_lines kernel/printk/printk_ringbuffer.c:1815 [inline]
 copy_data kernel/printk/printk_ringbuffer.c:1861 [inline]
 prb_read kernel/printk/printk_ringbuffer.c:1954 [inline]
 _prb_read_valid+0x65d/0x920 kernel/printk/printk_ringbuffer.c:2131
 prb_read_valid_info+0x66/0x90 kernel/printk/printk_ringbuffer.c:2238
 do_syslog+0x6e2/0x7e0 kernel/printk/printk.c:1826
 __do_sys_syslog kernel/printk/printk.c:1850 [inline]
 __se_sys_syslog kernel/printk/printk.c:1848 [inline]
 __x64_sys_syslog+0x41/0x50 kernel/printk/printk.c:1848
 x64_sys_call+0x2f45/0x3000 arch/x86/include/generated/asm/syscalls_64.h:104
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd2/0x200 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x3a -> 0x79

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 31361 Comm: syz.5.9228 Not tainted syzkaller #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
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