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BUG: KCSAN: data-race in sk_set_memalloc / sock_xmit

write to 0xffff88813b5eea08 of 4 bytes by task 2033 on cpu 1:
 sock_xmit+0x108/0x2d0 drivers/block/nbd.c:493
 nbd_read_stat+0xa6/0x9b0 drivers/block/nbd.c:692
 recv_work+0x4c/0x190 drivers/block/nbd.c:791
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read-write to 0xffff88813b5eea08 of 4 bytes by task 10561 on cpu 0:
 sk_set_memalloc+0x3a/0x50 net/core/sock.c:296
 nbd_start_device+0x469/0x720 drivers/block/nbd.c:1300
 nbd_start_device_ioctl drivers/block/nbd.c:1320 [inline]
 __nbd_ioctl drivers/block/nbd.c:1408 [inline]
 nbd_ioctl+0x2f9/0x780 drivers/block/nbd.c:1448
 __blkdev_driver_ioctl block/ioctl.c:228 [inline]
 blkdev_ioctl+0x1d6/0x3f0 block/ioctl.c:623
 block_ioctl+0x6d/0x80 fs/block_dev.c:1869
 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+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 10561 Comm: syz-executor.3 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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