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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff86e69890 of 8 bytes by task 7222 on cpu 1:
 data_alloc+0x31c/0x390 kernel/printk/printk_ringbuffer.c:1130
 prb_reserve+0x8d6/0xad0 kernel/printk/printk_ringbuffer.c:1727
 vprintk_store+0x54a/0x910 kernel/printk/printk.c:2354
 vprintk_emit+0x1a4/0x600 kernel/printk/printk.c:2455
 devkmsg_emit+0x7f/0xb0 kernel/printk/printk.c:733
 devkmsg_write+0x1e2/0x2b0 kernel/printk/printk.c:796
 do_iter_readv_writev+0x4fd/0x5a0 fs/read_write.c:-1
 vfs_writev+0x2e1/0x900 fs/read_write.c:1059
 do_writev+0xe9/0x210 fs/read_write.c:1105
 __do_sys_writev fs/read_write.c:1173 [inline]
 __se_sys_writev fs/read_write.c:1170 [inline]
 __x64_sys_writev+0x45/0x50 fs/read_write.c:1170
 x64_sys_call+0x1c0c/0x3020 arch/x86/include/generated/asm/syscalls_64.h:21
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff86e69890 of 8 bytes by task 15730 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+0x216/0xad0 kernel/printk/printk_ringbuffer.c:1677
 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
 logfc+0x2d5/0x2e0 fs/fs_context.c:-1
 shmem_parse_one+0xa7b/0xbc0 mm/shmem.c:-1
 vfs_parse_fs_param+0x142/0x2a0 fs/fs_context.c:132
 vfs_parse_fs_qstr fs/fs_context.c:173 [inline]
 vfs_parse_fs_string include/linux/fs_context.h:142 [inline]
 vfs_parse_monolithic_sep+0x1e6/0x270 fs/fs_context.c:213
 shmem_parse_monolithic+0x24/0x30 mm/shmem.c:4796
 parse_monolithic_mount_data+0x46/0x60 fs/fs_context.c:519
 do_new_mount+0x24e/0x8d0 fs/namespace.c:3830
 path_mount+0x4d0/0xbc0 fs/namespace.c:4154
 do_mount fs/namespace.c:4167 [inline]
 __do_sys_mount fs/namespace.c:4383 [inline]
 __se_sys_mount+0x28c/0x2e0 fs/namespace.c:4360
 __x64_sys_mount+0x67/0x80 fs/namespace.c:4360
 x64_sys_call+0x2d61/0x3020 arch/x86/include/generated/asm/syscalls_64.h:166
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xfffffffffffcf1b8 -> 0x000000000007e410

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 15730 Comm: syz.3.3241 Tainted: G        W           syzkaller #0 PREEMPT(full) 
Tainted: [W]=WARN
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
==================================================================
tmpfs: Bad value for 'mpol'