1WQ_HAS_SLEEPER(9)              Linux Networking              WQ_HAS_SLEEPER(9)
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NAME

6       wq_has_sleeper - check if there are any waiting processes
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SYNOPSIS

9       bool wq_has_sleeper(struct socket_wq * wq);
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ARGUMENTS

12       wq
13           struct socket_wq
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DESCRIPTION

16       Returns true if socket_wq has waiting processes
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18       The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the
19       memory barrier call. They were added due to the race found within the
20       tcp code.
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CONSIDER FOLLOWING TCP CODE PATHS

23       CPU1 CPU2
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25       sys_select receive packet ... ... __add_wait_queue update tp->rcv_nxt
26       ... ... tp->rcv_nxt check sock_def_readable ... { schedule
27       rcu_read_lock; wq = rcu_dereference(sk->sk_wq); if (wq &&
28       waitqueue_active(wq->wait)) wake_up_interruptible(wq->wait) ... }
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30       The race for tcp fires when the __add_wait_queue changes done by CPU1
31       stay in its cache, and so does the tp->rcv_nxt update on CPU2 side. The
32       CPU1 could then endup calling schedule and sleep forever if there are
33       no more data on the socket.
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36Kernel Hackers Manual 3.10         June 2019                 WQ_HAS_SLEEPER(9)
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