sk_has_sleeper man page on Scientific

sk_has_sleeper man page on Scientific

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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.	 November 2013		     SK_HAS_SLEEPER(9)

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