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BUG: KCSAN: data-race in data_alloc / prb_reserve

write to 0xffffffff876b5778 of 8 bytes by task 17800 on cpu 1:
 data_alloc+0x27e/0x2b0 kernel/printk/printk_ringbuffer.c:1085
 prb_reserve+0x8f0/0xbc0 kernel/printk/printk_ringbuffer.c:1549
 vprintk_store+0x3e9/0x9c0 kernel/printk/printk.c:2029
 vprintk_emit+0xca/0x3e0 kernel/printk/printk.c:2081
 vprintk_default+0x22/0x30 kernel/printk/printk.c:2115
 vprintk_func+0x11b/0x120 kernel/printk/printk_safe.c:401
 printk+0x62/0x82 kernel/printk/printk.c:2146
 set_capacity_and_notify+0x141/0x1a0 block/genhd.c:75
 loop_set_size drivers/block/loop.c:254 [inline]
 loop_configure+0xafc/0xcb0 drivers/block/loop.c:1161
 lo_ioctl+0x555/0x11f0 drivers/block/loop.c:1689
 blkdev_ioctl+0x1d0/0x3c0 block/ioctl.c:583
 block_ioctl+0x6d/0x80 fs/block_dev.c:1667
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff876b5778 of 8 bytes by task 17802 on cpu 0:
 desc_read kernel/printk/printk_ringbuffer.c:477 [inline]
 desc_push_tail kernel/printk/printk_ringbuffer.c:771 [inline]
 desc_reserve kernel/printk/printk_ringbuffer.c:917 [inline]
 prb_reserve+0x20c/0xbc0 kernel/printk/printk_ringbuffer.c:1499
 vprintk_store+0x3e9/0x9c0 kernel/printk/printk.c:2029
 vprintk_emit+0xca/0x3e0 kernel/printk/printk.c:2081
 vprintk_default+0x22/0x30 kernel/printk/printk.c:2115
 vprintk_func+0x11b/0x120 kernel/printk/printk_safe.c:401
 printk+0x62/0x82 kernel/printk/printk.c:2146
 set_capacity_and_notify+0x141/0x1a0 block/genhd.c:75
 loop_set_size drivers/block/loop.c:254 [inline]
 loop_configure+0xafc/0xcb0 drivers/block/loop.c:1161
 lo_ioctl+0x555/0x11f0 drivers/block/loop.c:1689
 blkdev_ioctl+0x1d0/0x3c0 block/ioctl.c:583
 block_ioctl+0x6d/0x80 fs/block_dev.c:1667
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 17802 Comm: syz-executor.2 Tainted: G        W         5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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