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)
