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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>sk_has_sleeper</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Linux Networking and Network Devices APIs" /><link rel="up" href="ch01s02.html" title="Socket Buffer Functions" /><link rel="prev" href="re51.html" title="sk_has_allocations" /><link rel="next" href="re53.html" title="sock_poll_wait" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span>sk_has_sleeper</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re51.html">Prev</a> </td><th width="60%" align="center">Socket Buffer Functions</th><td width="20%" align="right"> <a accesskey="n" href="re53.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="sk_has_sleeper"><a id="API-sk-has-sleeper"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>sk_has_sleeper —
check if there are any waiting processes
</p></div><div class="refsynopsisdiv" title="Synopsis"><h2>Synopsis</h2><div class="funcsynopsis"><table border="0" summary="Function synopsis" cellspacing="0" cellpadding="0" class="funcprototype-table"><tr><td><code class="funcdef">int <b class="fsfunc">sk_has_sleeper </b>(</code></td><td>struct sock * <var class="pdparam">sk</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id2691407"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>sk</code></em></span></dt><dd><p>
socket
</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id2691432"></a><h2>Description</h2><p>
Returns true if socket has waiting processes
</p><p>
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.
</p></div><div class="refsect1" title="Consider following tcp code paths"><a id="id2691448"></a><h2>Consider following tcp code paths</h2><p>
</p><p>
CPU1 CPU2
</p><p>
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)
...
}
</p><p>
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.
</p><p>
The sk_has_sleeper is always called right after a call to read_lock, so we
can use smp_mb__after_lock barrier.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="re51.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ch01s02.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="re53.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span>sk_has_allocations</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span>sock_poll_wait</span></td></tr></table></div></body></html>
