BUG: KCSAN: data-race in desc_read_finalized_seq / prb_reserve

write to 0xffffffff8689c070 of 8 bytes by task 3395 on cpu 0:
 prb_reserve+0x750/0xb80 kernel/printk/printk_ringbuffer.c:-1
 vprintk_store+0x552/0x860 kernel/printk/printk.c:2326
 vprintk_emit+0x1a0/0x6c0 kernel/printk/printk.c:2426
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2465
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x7f/0xb0 kernel/printk/printk.c:2475
 printk_stack_address arch/x86/kernel/dumpstack.c:72 [inline]
 show_trace_log_lvl+0x300/0x3d0 arch/x86/kernel/dumpstack.c:284
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0xf6/0x150 lib/dump_stack.c:120
 dump_stack+0x15/0x1a lib/dump_stack.c:129
 __getblk_slow fs/buffer.c:1120 [inline]
 bdev_getblk+0x34f/0x3b0 fs/buffer.c:1431
 sb_getblk_gfp include/linux/buffer_head.h:391 [inline]
 __read_extent_tree_block+0xb0/0x480 fs/ext4/extents.c:565
 ext4_find_extent+0x510/0x7e0 fs/ext4/extents.c:938
 ext4_ext_map_blocks+0x121/0x36a0 fs/ext4/extents.c:4205
 ext4_map_create_blocks fs/ext4/inode.c:520 [inline]
 ext4_map_blocks+0x5c6/0xd00 fs/ext4/inode.c:706
 mpage_map_one_extent fs/ext4/inode.c:2224 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2277 [inline]
 ext4_do_writepages+0x14ef/0x21d0 fs/ext4/inode.c:2739
 ext4_writepages+0x183/0x320 fs/ext4/inode.c:2829
 do_writepages+0x1d8/0x480 mm/page-writeback.c:2656
 __writeback_single_inode+0x89/0x850 fs/fs-writeback.c:1680
 writeback_sb_inodes+0x458/0xa10 fs/fs-writeback.c:1976
 wb_writeback+0x264/0x650 fs/fs-writeback.c:2156
 wb_do_writeback fs/fs-writeback.c:2303 [inline]
 wb_workfn+0x1aa/0x960 fs/fs-writeback.c:2343
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0x4de/0xa20 kernel/workqueue.c:3319
 worker_thread+0x52c/0x710 kernel/workqueue.c:3400
 kthread+0x4b7/0x540 kernel/kthread.c:464
 ret_from_fork+0x4b/0x60 arch/x86/kernel/process.c:153
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffffffff8689c070 of 8 bytes by task 2993 on cpu 1:
 desc_read kernel/printk/printk_ringbuffer.c:486 [inline]
 desc_read_finalized_seq+0x163/0x280 kernel/printk/printk_ringbuffer.c:1874
 prb_read kernel/printk/printk_ringbuffer.c:1922 [inline]
 _prb_read_valid+0x142/0x710 kernel/printk/printk_ringbuffer.c:2113
 prb_read_valid+0x42/0x70 kernel/printk/printk_ringbuffer.c:2185
 syslog_print+0x242/0x540 kernel/printk/printk.c:1650
 do_syslog+0x3ae/0x810 kernel/printk/printk.c:1785
 __do_sys_syslog kernel/printk/printk.c:1877 [inline]
 __se_sys_syslog kernel/printk/printk.c:1875 [inline]
 __x64_sys_syslog+0x43/0x50 kernel/printk/printk.c:1875
 x64_sys_call+0x2b8f/0x2e10 arch/x86/include/generated/asm/syscalls_64.h:104
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xc9/0x1a0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0x0000000000008070 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 2993 Comm: klogd Not tainted 6.15.0-rc2-syzkaller-00278-gfc96b232f8e7 #0 PREEMPT(voluntary) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
==================================================================