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<!--$Id: deadlock.so,v 10.20 2002/06/24 14:51:07 bostic Exp $-->
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<title>Berkeley DB Reference Guide: Deadlock detection</title>
<meta name="description" content="Berkeley DB: An embedded database programmatic toolkit.">
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<td><h3><dl><dt>Berkeley DB Reference Guide:<dd>Berkeley DB Transactional Data Store Applications</dl></h3></td>
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<p>
<h3 align=center>Deadlock detection</h3>
<p>The first component of the infrastructure, <i>deadlock
detection</i>, is not so much a requirement specific to
transaction-protected applications, but instead is necessary for almost
all applications in which more than a single thread of control will be
accessing the database at one time. Even when Berkeley DB automatically
handles database locking, it is normally possible for deadlock to occur.
Because the underlying database access methods may update multiple pages
during a single Berkeley DB API call, deadlock is possible even when threads
of control are making only single update calls into the database. The
exception to this rule is when all the threads of control accessing the
database are read-only or when the Berkeley DB Concurrent Data Store product is used; the Berkeley DB Concurrent Data Store
product guarantees deadlock-free operation at the expense of reduced
concurrency.</p>
<p>When the deadlock occurs, two (or more) threads of control each request
additional locks that can never be granted because one of the threads
of control waiting holds the requested resource. For example, consider
two processes: A and B. Let's say that A obtains a write lock on item
X, and B obtains a write lock on item Y. Then, A requests a lock on Y,
and B requests a lock on X. A will wait until resource Y becomes
available and B will wait until resource X becomes available.
Unfortunately, because both A and B are waiting, neither will release
the locks they hold and neither will ever obtain the resource on which
it is waiting. For another example, consider two transactions, A and
B, each of which may want to modify item X. Assume that transaction A
obtains a read lock on X and confirms that a modification is needed.
Then it is descheduled and the thread containing transaction B runs.
At that time, transaction B obtains a read lock on X and confirms that
it also wants to make a modification. Both transactions A and B will
block when they attempt to upgrade their read locks to write locks
because the other already holds a read lock. This is a deadlock.
Transaction A cannot make forward progress until Transaction B releases
its read lock on X, but Transaction B cannot make forward progress until
Transaction A releases its read lock on X.</p>
<p>In order to detect that deadlock has happened, a separate process or
thread must review the locks currently held in the database. If
deadlock has occurred, a victim must be selected, and that victim will
then return the error <a href="../../ref/program/errorret.html#DB_LOCK_DEADLOCK">DB_LOCK_DEADLOCK</a> from whatever Berkeley DB call
it was making. Berkeley DB provides a separate UNIX-style utility that can
be used to perform this deadlock detection, named <a href="../../utility/db_deadlock.html">db_deadlock</a>.
Alternatively, applications can create their own deadlock utility or
thread using the underlying <a href="../../api_c/lock_detect.html">DB_ENV->lock_detect</a> function, or specify
that Berkeley DB run the deadlock detector internally whenever there is a
conflict over a lock (see <a href="../../api_c/env_set_lk_detect.html">DB_ENV->set_lk_detect</a> for more
information). The following code fragment does the latter:</p>
<blockquote><pre>void
env_open(DB_ENV **dbenvp)
{
DB_ENV *dbenv;
int ret;
<p>
/* Create the environment handle. */
if ((ret = db_env_create(&dbenv, 0)) != 0) {
fprintf(stderr,
"txnapp: db_env_create: %s\n", db_strerror(ret));
exit (1);
}
<p>
/* Set up error handling. */
dbenv->set_errpfx(dbenv, "txnapp");
dbenv->set_errfile(dbenv, stderr);
<p>
<b> /* Do deadlock detection internally. */
if ((ret = dbenv->set_lk_detect(dbenv, DB_LOCK_DEFAULT)) != 0) {
dbenv->err(dbenv, ret, "set_lk_detect: DB_LOCK_DEFAULT");
exit (1);
}</b>
<p>
/*
* Open a transactional environment:
* create if it doesn't exist
* free-threaded handle
* run recovery
* read/write owner only
*/
if ((ret = dbenv->open(dbenv, ENV_DIRECTORY,
DB_CREATE | DB_INIT_LOCK | DB_INIT_LOG |
DB_INIT_MPOOL | DB_INIT_TXN | DB_RECOVER | DB_THREAD,
S_IRUSR | S_IWUSR)) != 0) {
dbenv->err(dbenv, ret, "dbenv->open: %s", ENV_DIRECTORY);
exit (1);
}
<p>
*dbenvp = dbenv;
}</pre></blockquote>
<p>Deciding how often to run the deadlock detector and which of the
deadlocked transactions will be forced to abort when the deadlock is
detected is a common tuning parameter for Berkeley DB applications.</p>
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