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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff86c7b788 of 8 bytes by task 7557 on cpu 1:
 data_alloc+0x31c/0x390 kernel/printk/printk_ringbuffer.c:1130
 prb_reserve+0x8d7/0xae0 kernel/printk/printk_ringbuffer.c:1724
 vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x600 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
 buffer_io_error fs/buffer.c:131 [inline]
 end_buffer_async_read fs/buffer.c:272 [inline]
 end_buffer_async_read_io+0x232/0x250 fs/buffer.c:377
 end_bio_bh_io_sync+0x79/0xa0 fs/buffer.c:2773
 bio_endio+0x38a/0x420 block/bio.c:1792
 blk_mq_submit_bio+0x3d2/0x1150 block/blk-mq.c:-1
 __submit_bio+0x103/0x4f0 block/blk-core.c:635
 __submit_bio_noacct_mq block/blk-core.c:722 [inline]
 submit_bio_noacct_nocheck+0x152/0x640 block/blk-core.c:753
 submit_bio_noacct+0x746/0x9b0 block/blk-core.c:884
 submit_bio+0x2a4/0x2c0 block/blk-core.c:926
 blk_crypto_submit_bio include/linux/blk-crypto.h:203 [inline]
 submit_bh_wbc+0x2e0/0x320 fs/buffer.c:2821
 submit_bh fs/buffer.c:2826 [inline]
 block_read_full_folio+0x658/0x6a0 fs/buffer.c:2458
 blkdev_read_folio+0x1c/0x30 block/fops.c:494
 filemap_read_folio mm/filemap.c:2496 [inline]
 do_read_cache_folio+0x19b/0x600 mm/filemap.c:4096
 read_cache_folio+0x3c/0x50 mm/filemap.c:4128
 read_mapping_folio include/linux/pagemap.h:1028 [inline]
 read_part_sector+0x6f/0x150 block/partitions/core.c:723
 msdos_partition+0x82/0x2050 block/partitions/msdos.c:592
 check_partition block/partitions/core.c:142 [inline]
 blk_add_partitions block/partitions/core.c:590 [inline]
 bdev_disk_changed+0x511/0xd40 block/partitions/core.c:694
 loop_reread_partitions+0x48/0xb0 drivers/block/loop.c:447
 loop_configure+0x981/0x9c0 drivers/block/loop.c:1096
 lo_ioctl+0x5aa/0x13a0 drivers/block/loop.c:1542
 blkdev_ioctl+0x387/0x460 block/ioctl.c:804
 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+0x1563/0x3020 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff86c7b788 of 8 bytes by task 7560 on cpu 0:
 desc_read kernel/printk/printk_ringbuffer.c:496 [inline]
 desc_push_tail kernel/printk/printk_ringbuffer.c:792 [inline]
 desc_reserve kernel/printk/printk_ringbuffer.c:938 [inline]
 prb_reserve+0x207/0xae0 kernel/printk/printk_ringbuffer.c:1674
 vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x600 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
 set_capacity_and_notify+0x1dd/0x230 block/genhd.c:93
 loop_set_size drivers/block/loop.c:220 [inline]
 loop_set_status+0x556/0x6a0 drivers/block/loop.c:1271
 loop_set_status_old drivers/block/loop.c:1362 [inline]
 lo_ioctl+0x799/0x13a0 drivers/block/loop.c:1551
 blkdev_ioctl+0x387/0x460 block/ioctl.c:804
 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+0x1563/0x3020 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x370 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xffffffffffffb5d8 -> 0x00000000000c8ea0

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 7560 Comm: syz.5.16467 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
==================================================================
loop9: detected capacity change from 7 to 0