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BUG: KCSAN: data-race in _prb_read_valid / prb_reserve

write to 0xffffffff86ed8c80 of 88 bytes by task 19960 on cpu 0:
 prb_reserve+0x6fb/0xaf0 kernel/printk/printk_ringbuffer.c:1693
 vprintk_store+0x54e/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x610 kernel/printk/printk.c:2455
 vprintk_default+0x26/0x30 kernel/printk/printk.c:2494
 vprintk+0x1d/0x30 kernel/printk/printk_safe.c:82
 _printk+0x79/0xa0 kernel/printk/printk.c:2504
 _fat_msg+0xb0/0xe0 fs/fat/misc.c:62
 __fat_fs_error+0x183/0x1e0 fs/fat/misc.c:31
 fat_bmap_cluster fs/fat/cache.c:298 [inline]
 fat_get_mapped_cluster+0x235/0x250 fs/fat/cache.c:315
 fat_bmap+0x250/0x280 fs/fat/cache.c:379
 __fat_get_block fs/fat/inode.c:129 [inline]
 fat_get_block+0xd3/0x5e0 fs/fat/inode.c:190
 block_read_full_folio+0x1eb/0x690 fs/buffer.c:2308
 do_mpage_readpage+0x29f/0xe80 fs/mpage.c:313
 mpage_read_folio+0xbd/0x130 fs/mpage.c:394
 fat_read_folio+0x1c/0x30 fs/fat/inode.c:205
 filemap_read_folio+0x2e/0x110 mm/filemap.c:2520
 filemap_fault+0x5a4/0xb60 mm/filemap.c:3689
 __do_fault mm/memory.c:5565 [inline]
 do_read_fault mm/memory.c:6000 [inline]
 do_fault mm/memory.c:6134 [inline]
 do_pte_missing mm/memory.c:4700 [inline]
 handle_pte_fault mm/memory.c:6558 [inline]
 __handle_mm_fault mm/memory.c:6707 [inline]
 handle_mm_fault+0x2310/0x3bb0 mm/memory.c:6876
 faultin_page mm/gup.c:1126 [inline]
 __get_user_pages+0x12e9/0x1eb0 mm/gup.c:1428
 populate_vma_page_range mm/gup.c:1860 [inline]
 __mm_populate+0x242/0x390 mm/gup.c:1963
 mm_populate include/linux/mm.h:4288 [inline]
 vm_mmap_pgoff+0x23b/0x2d0 mm/util.c:586
 ksys_mmap_pgoff+0x250/0x310 mm/mmap.c:619
 x64_sys_call+0x23b3/0x2550 arch/x86/include/generated/asm/syscalls_64.h:10
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff86ed8c80 of 8 bytes by task 15 on cpu 1:
 desc_read kernel/printk/printk_ringbuffer.c:500 [inline]
 prb_first_seq kernel/printk/printk_ringbuffer.c:2017 [inline]
 _prb_read_valid+0x325/0x930 kernel/printk/printk_ringbuffer.c:2174
 prb_read_valid+0x3c/0x60 kernel/printk/printk_ringbuffer.c:2245
 printk_get_next_message+0xea/0x550 kernel/printk/printk.c:3052
 nbcon_emit_next_record+0x4af/0x1bc0 kernel/printk/nbcon.c:1017
 nbcon_emit_one kernel/printk/nbcon.c:1157 [inline]
 nbcon_kthread_func+0x3fd/0x690 kernel/printk/nbcon.c:1271
 kthread+0x223/0x270 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x0000000000002c70 -> 0x0000000000004c70

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 15 Comm: pr/ttyS0 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
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