suse man page for sk_has_sleeper

SK_HAS_SLEEPER(9)						 Linux Networking						 SK_HAS_SLEEPER(9)

NAME
       sk_has_sleeper - check if there are any waiting processes

SYNOPSIS
       int sk_has_sleeper(struct sock * sk);

ARGUMENTS
       sk
	   socket

DESCRIPTION
       Returns true if socket has waiting processes

       The purpose of the sk_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 ... if
       (sk->sk_sleep && waitqueue_active(sk->sk_sleep)) wake_up_interruptible(sk->sk_sleep) ... }

       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.

       The sk_has_sleeper is always called right after a call to read_lock, so we can use smp_mb__after_lock barrier.

COPYRIGHT
Kernel Hackers Manual 2.6.					     July 2010							 SK_HAS_SLEEPER(9)