'\" t
.\"     Title: skwq_has_sleeper
.\"    Author: [FIXME: author] [see http://docbook.sf.net/el/author]
.\" Generator: DocBook XSL Stylesheets v1.79.1 <http://docbook.sf.net/>
.\"      Date: January 2017
.\"    Manual: Linux Networking
.\"    Source: Kernel Hackers Manual 4.8.15
.\"  Language: English
.\"
.TH "SKWQ_HAS_SLEEPER" "9" "January 2017" "Kernel Hackers Manual 4\&.8\&." "Linux Networking"
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.SH "NAME"
skwq_has_sleeper \- check if there are any waiting processes
.SH "SYNOPSIS"
.HP \w'bool\ skwq_has_sleeper('u
.BI "bool skwq_has_sleeper(struct\ socket_wq\ *\ " "wq" ");"
.SH "ARGUMENTS"
.PP
\fIwq\fR
.RS 4
struct socket_wq
.RE
.SH "DESCRIPTION"
.PP
Returns true if socket_wq has waiting processes
.PP
The purpose of the skwq_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\&.
.PP
Consider following tcp code paths:
.PP
CPU1 CPU2
.PP
sys_select receive packet \&.\&.\&. \&.\&.\&. __add_wait_queue update tp\->rcv_nxt \&.\&.\&. \&.\&.\&. tp\->rcv_nxt check sock_def_readable \&.\&.\&. { schedule
\fBrcu_read_lock\fR; wq = rcu_dereference(sk\->sk_wq); if (wq && waitqueue_active(wq\->wait)) wake_up_interruptible(wq\->wait) \&.\&.\&. }
.PP
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\&.
.SH "COPYRIGHT"
.br