centos man page for wq_has_sleeper

WQ_HAS_SLEEPER(9)						 Linux Networking						 WQ_HAS_SLEEPER(9)

NAME
       wq_has_sleeper - check if there are any waiting processes

SYNOPSIS
       bool wq_has_sleeper(struct socket_wq * wq);

ARGUMENTS
       wq
	   struct socket_wq

DESCRIPTION
       Returns true if socket_wq has waiting processes

       The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the memory barrier call. They were added due to the race found within the
       tcp code.

CONSIDER FOLLOWING TCP CODE PATHS
       CPU1 CPU2

       sys_select receive packet ... ... __add_wait_queue update tp->rcv_nxt ... ... tp->rcv_nxt check sock_def_readable ... { schedule
       rcu_read_lock; wq = rcu_dereference(sk->sk_wq); if (wq && waitqueue_active(wq->wait)) wake_up_interruptible(wq->wait) ... }

       The race for tcp fires when the __add_wait_queue changes done by CPU1 stay in its cache, and so does the tp->rcv_nxt update on CPU2 side.
       The CPU1 could then endup calling schedule and sleep forever if there are no more data on the socket.

COPYRIGHT
Kernel Hackers Manual 3.10					     June 2014							 WQ_HAS_SLEEPER(9)