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BUG: KCSAN: data-race in sk_set_memalloc / sock_xmit

write to 0xffff8880b11f5608 of 4 bytes by task 2337 on cpu 1:
 sock_xmit+0x117/0x300 drivers/block/nbd.c:492
 nbd_read_stat+0xa6/0xa40 drivers/block/nbd.c:691
 recv_work+0x4c/0x190 drivers/block/nbd.c:790
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff8880b11f5608 of 4 bytes by task 21156 on cpu 0:
 sk_set_memalloc+0x34/0x60 net/core/sock.c:296
 nbd_start_device+0x454/0x710 drivers/block/nbd.c:1299
 nbd_start_device_ioctl drivers/block/nbd.c:1319 [inline]
 __nbd_ioctl drivers/block/nbd.c:1407 [inline]
 nbd_ioctl+0x2f9/0x770 drivers/block/nbd.c:1447
 __blkdev_driver_ioctl block/ioctl.c:224 [inline]
 blkdev_ioctl+0x1cf/0x410 block/ioctl.c:620
 block_ioctl+0x6d/0x80 fs/block_dev.c:1871
 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: 21156 Comm: syz-executor.3 Not tainted 5.9.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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