BUG: KCSAN: data-race in _prb_read_valid / prb_reserve

write to 0xffffffff868f6280 of 88 bytes by task 26734 on cpu 1:
 prb_reserve+0x696/0xaf0 kernel/printk/printk_ringbuffer.c:1635
 vprintk_store+0x56d/0x860 kernel/printk/printk.c:2326
 vprintk_emit+0x178/0x650 kernel/printk/printk.c:2426
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2465
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2475
 printk_stack_address arch/x86/kernel/dumpstack.c:70 [inline]
 show_trace_log_lvl+0x4e3/0x560 arch/x86/kernel/dumpstack.c:282
 __dump_stack+0x1d/0x30 lib/dump_stack.c:94
 dump_stack_lvl+0xe8/0x140 lib/dump_stack.c:120
 dump_stack+0x15/0x1b lib/dump_stack.c:129
 fail_dump lib/fault-inject.c:73 [inline]
 should_fail_ex+0x265/0x280 lib/fault-inject.c:174
 should_fail+0xb/0x20 lib/fault-inject.c:184
 should_fail_usercopy+0x1a/0x20 lib/fault-inject-usercopy.c:37
 _inline_copy_to_user include/linux/uaccess.h:193 [inline]
 _copy_to_user+0x20/0xa0 lib/usercopy.c:26
 copy_to_user include/linux/uaccess.h:225 [inline]
 simple_read_from_buffer+0xb5/0x130 fs/libfs.c:1135
 proc_fail_nth_read+0x10e/0x150 fs/proc/base.c:1488
 vfs_read+0x1a8/0x770 fs/read_write.c:570
 ksys_read+0xda/0x1a0 fs/read_write.c:715
 __do_sys_read fs/read_write.c:724 [inline]
 __se_sys_read fs/read_write.c:722 [inline]
 __x64_sys_read+0x40/0x50 fs/read_write.c:722
 x64_sys_call+0x27bc/0x2ff0 arch/x86/include/generated/asm/syscalls_64.h:1
 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 0xffffffff868f6280 of 8 bytes by task 26723 on cpu 0:
 desc_read kernel/printk/printk_ringbuffer.c:486 [inline]
 prb_first_seq kernel/printk/printk_ringbuffer.c:1957 [inline]
 _prb_read_valid+0x383/0x920 kernel/printk/printk_ringbuffer.c:2114
 desc_update_last_finalized kernel/printk/printk_ringbuffer.c:1515 [inline]
 prb_final_commit+0x136/0x1e0 kernel/printk/printk_ringbuffer.c:1782
 vprintk_store+0x741/0x860 kernel/printk/printk.c:2352
 vprintk_emit+0x178/0x650 kernel/printk/printk.c:2426
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2465
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2475
 set_capacity_and_notify+0x14c/0x1f0 block/genhd.c:93
 loop_set_size+0x2e/0x70 drivers/block/loop.c:214
 loop_configure+0x825/0x9c0 drivers/block/loop.c:1071
 lo_ioctl+0x1e1/0x1240 drivers/block/loop.c:1526
 blkdev_ioctl+0x34f/0x440 block/ioctl.c:705
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:598 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:584
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:584
 x64_sys_call+0x1816/0x2ff0 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

value changed: 0x0000000000003126 -> 0x0000000000005126

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 26723 Comm: syz.0.7584 Tainted: G        W           syzkaller #0 PREEMPT(voluntary) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/12/2025
==================================================================
EXT4-fs: Ignoring removed nobh option
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #3: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0): ext4_dirty_inode:6538: inode #3: comm syz.0.7584: mark_inode_dirty error
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #3: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0): __ext4_ext_dirty:206: inode #3: comm syz.0.7584: mark_inode_dirty error
EXT4-fs error (device loop0): ext4_acquire_dquot:6937: comm syz.0.7584: Failed to acquire dquot type 0
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #16: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0): ext4_dirty_inode:6538: inode #16: comm syz.0.7584: mark_inode_dirty error
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #16: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0): __ext4_ext_dirty:206: inode #16: comm syz.0.7584: mark_inode_dirty error
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #16: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0) in ext4_orphan_del:305: Corrupt filesystem
EXT4-fs error (device loop0): ext4_do_update_inode:5653: inode #16: comm syz.0.7584: corrupted inode contents
EXT4-fs error (device loop0): ext4_truncate:4666: inode #16: comm syz.0.7584: mark_inode_dirty error
EXT4-fs error (device loop0) in ext4_process_orphan:347: Corrupt filesystem
EXT4-fs (loop0): 1 truncate cleaned up
EXT4-fs (loop0): mounted filesystem 00000000-0000-0000-0000-000000000000 r/w without journal. Quota mode: writeback.
EXT4-fs (loop0): unmounting filesystem 00000000-0000-0000-0000-000000000000.