Web UI JSP pages thread safety issue
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Key: HDFS-1758
URL: https://issues.apache.org/jira/browse/HDFS-1758
Project: Hadoop HDFS
Issue Type: Bug
Components: tools
Environment: branch-20-security
Reporter: Tanping Wang
Assignee: Tanping Wang
Priority: Minor
The set of JSP pages that web UI uses are not thread safe. We have observed
some problems when requesting Live/Dead/Decommissioning pages from the web UI,
incorrect page is displayed. To be more specific, requesting Dead node list
page, sometimes, Live node page is returned. Requesting decommissioning page,
sometimes, dead page is returned.
The root cause of this problem is that JSP pages is not thread safe by default.
When multiple requests come in, each request is assigned a different thread,
multiple threads access the same instance of the servlet class resulted from a
JSP page. A class variable is shared by multiple threads. In code out of 20
branches, for example, dfsnodelist.jsp has
{code}
int rowNum = 0;
int colNum = 0;
String sorterField = null;
String sorterOrder = null;
String whatNodes = "LIVE";
{code}
declared as class level variables. ( These set of variables are declared
within <%! code %> directives which made them class members. ) Multiple
threads share the same set of class member variables, one request would step
anther's toe.
However, due to the JSP code refactor, HADOOP-5857, all of these class member
variables are moved to become function local variables. So this bug does not
appear in Apache trunk. We have proposed to take a simple fix to fix this bug
in 20 branch alone, to be more specific, branch-0.20-security.
The simple fix is to add jsp ThreadSafe="false" directive into the related JSP
pages to make them thread safe, i.e. only on request is processed at each time.
We did evaluate the thread safety issue for other JSP pages on trunk, we
noticed a potential problem that is when we retrieving some statistics from
namenode, for example, we make the call
NamenodeJspHelper.getInodeLimitText(fsn);
in dfshealth.jsp, which eventuality is
{code}
static String getInodeLimitText(FSNamesystem fsn) {
long inodes = fsn.dir.totalInodes();
long blocks = fsn.getBlocksTotal();
long maxobjects = fsn.getMaxObjects();
....
{code}
some of the function calls are already guarded by readwritelock, e.g.
dir.totalInodes, but others are not. As a result of this, the web ui results
are not 100% thread safe. But after evaluating the prons and cons of adding a
giant lock into the JSP pages, we decide not to issue FSNamesystem ReadWrite
locks into JSPs.
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