<?xml version="1.0" encoding="ANSI_X3.4-1968" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=ANSI_X3.4-1968" /><title>d_lookup</title><meta name="generator" content="DocBook XSL Stylesheets V1.75.2" /><link rel="home" href="index.html" title="Linux Filesystems API" /><link rel="up" href="ch01s02.html" title="The Directory Cache" /><link rel="prev" href="re15.html" title="d_add_ci" /><link rel="next" href="re17.html" title="d_validate" /></head><body><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="3" align="center"><span>d_lookup</span></th></tr><tr><td width="20%" align="left"><a accesskey="p" href="re15.html">Prev</a> </td><th width="60%" align="center">The Directory Cache</th><td width="20%" align="right"> <a accesskey="n" href="re17.html">Next</a></td></tr></table><hr /></div><div class="refentry" title="d_lookup"><a id="API-d-lookup"></a><div class="titlepage"></div><div class="refnamediv"><h2>Name</h2><p>d_lookup —
search for a dentry
</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">struct dentry * <b class="fsfunc">d_lookup </b>(</code></td><td>struct dentry * <var class="pdparam">parent</var>, </td></tr><tr><td> </td><td>struct qstr * <var class="pdparam">name</var><code>)</code>;</td></tr></table><div class="funcprototype-spacer"> </div></div></div><div class="refsect1" title="Arguments"><a id="id3004287"></a><h2>Arguments</h2><div class="variablelist"><dl><dt><span class="term"><em class="parameter"><code>parent</code></em></span></dt><dd><p>
parent dentry
</p></dd><dt><span class="term"><em class="parameter"><code>name</code></em></span></dt><dd><p>
qstr of name we wish to find
</p></dd></dl></div></div><div class="refsect1" title="Description"><a id="id3004329"></a><h2>Description</h2><p>
Searches the children of the parent dentry for the name in question. If
the dentry is found its reference count is incremented and the dentry
is returned. The caller must use dput to free the entry when it has
finished using it. <code class="constant">NULL</code> is returned on failure.
</p><p>
__d_lookup is dcache_lock free. The hash list is protected using RCU.
Memory barriers are used while updating and doing lockless traversal.
To avoid races with d_move while rename is happening, d_lock is used.
</p><p>
Overflows in <code class="function">memcmp</code>, while d_move, are avoided by keeping the length
and name pointer in one structure pointed by d_qstr.
</p><p>
<code class="function">rcu_read_lock</code> and <code class="function">rcu_read_unlock</code> are used to disable preemption while
lookup is going on.
</p><p>
The dentry unused LRU is not updated even if lookup finds the required dentry
in there. It is updated in places such as prune_dcache, shrink_dcache_sb,
select_parent and __dget_locked. This laziness saves lookup from dcache_lock
acquisition.
</p><p>
<code class="function">d_lookup</code> is protected against the concurrent renames in some unrelated
directory using the seqlockt_t rename_lock.
</p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="re15.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="re17.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top"><span>d_add_ci</span> </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> <span>d_validate</span></td></tr></table></div></body></html>
