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Deadlock debugging
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## Deadlock debugging

An occasional debugging problem in Berkeley DB applications is unresolvable deadlock. The output of the -Co flags of the db_stat utility can be used to detect and debug these problems. The following is a typical example of the output of this utility:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
           1  READ         1  HELD        a.db                handle   0
    80000004  WRITE        1  HELD        a.db                page     3

In this example, we have opened a database and stored a single key/data pair in it. Because we have a database handle open, we have a read lock on that database handle. The database handle lock is the read lock labeled handle. (We can normally ignore handle locks for the purposes of database debugging, as they will only conflict with other handle operations, for example, an attempt to remove the database will block because we are holding the handle locked, but reading and writing the database will not conflict with the handle lock.)

It is important to note that locker IDs are 32-bit unsigned integers, and are divided into two name spaces. Locker IDs with the high bit set (that is, values 80000000 or higher), are locker IDs associated with transactions. Locker IDs without the high bit set are locker IDs that are not associated with a transaction. Locker IDs associated with transactions map one-to-one with the transaction, that is, a transaction never has more than a single locker ID, and all of the locks acquired by the transaction will be acquired on behalf of the same locker ID.

We also hold a write lock on the database page where we stored the new key/data pair. The page lock is labeled page and is on page number 3. If we were to put an additional key/data pair in the database, we would see the following output:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
    80000004  WRITE        2  HELD        a.db                page     3
           1  READ         1  HELD        a.db                handle   0

That is, we have acquired a second reference count to page number 3, but have not acquired any new locks. If we add an entry to a different page in the database, we would acquire additional locks:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
           1  READ         1  HELD        a.db                handle   0
    80000004  WRITE        2  HELD        a.db                page     3
    80000004  WRITE        1  HELD        a.db                page     2

Here's a simple example of one lock blocking another one:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
    80000004  WRITE        1  HELD        a.db                page     2
    80000005  WRITE        1  WAIT        a.db                page     2
           1  READ         1  HELD        a.db                handle   0
    80000004  READ         1  HELD        a.db                page     1

In this example, there are two different transactional lockers (80000004 and 80000005). Locker 80000004 is holding a write lock on page 2, and locker 80000005 is waiting for a write lock on page 2. This is not a deadlock, because locker 80000004 is not blocked on anything. Presumably, the thread of control using locker 80000004 will proceed, eventually release its write lock on page 2, at which point the thread of control using locker 80000005 can also proceed, acquiring a write lock on page 2.

If lockers 80000004 and 80000005 are not in different threads of control, the result would be self deadlock. Self deadlock is not a true deadlock, and won't be detected by the Berkeley DB deadlock detector. It's not a true deadlock because, if work could continue to be done on behalf of locker 80000004, then the lock would eventually be released, and locker 80000005 could acquire the lock and itself proceed. So, the key element is that the thread of control holding the lock cannot proceed because it is the same thread as is blocked waiting on the lock.

Here's an example of three transactions reaching true deadlock. First, three different threads of control opened the database, acquiring three database handle read locks.

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
           1  READ         1  HELD        a.db                handle   0
           3  READ         1  HELD        a.db                handle   0
           5  READ         1  HELD        a.db                handle   0

The three threads then each began a transaction, and put a key/data pair on a different page:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
    80000008  WRITE        1  HELD        a.db                page     4
           1  READ         1  HELD        a.db                handle   0
           3  READ         1  HELD        a.db                handle   0
           5  READ         1  HELD        a.db                handle   0
    80000006  READ         1  HELD        a.db                page     1
    80000007  READ         1  HELD        a.db                page     1
    80000008  READ         1  HELD        a.db                page     1
    80000006  WRITE        1  HELD        a.db                page     2
    80000007  WRITE        1  HELD        a.db                page     3

The thread using locker 80000006 put a new key/data pair on page 2, the thread using locker 80000007, on page 3, and the thread using locker 80000008 on page 4. Because the database is a 2-level Btree, the tree was searched, and so each transaction acquired a read lock on the Btree root page (page 1) as part of this operation.

The three threads then each attempted to put a second key/data pair on a page currently locked by another thread. The thread using locker 80000006 tried to put a key/data pair on page 3, the thread using locker 80000007 on page 4, and the thread using locker 80000008 on page 2:

    Locks grouped by object
    Locker    Mode    Count   Status      ----------- Object ----------
    80000008  WRITE        1  HELD        a.db                page     4
    80000007  WRITE        1  WAIT        a.db                page     4
           1  READ         1  HELD        a.db                handle   0
           3  READ         1  HELD        a.db                handle   0
           5  READ         1  HELD        a.db                handle   0
    80000006  READ         2  HELD        a.db                page     1
    80000007  READ         2  HELD        a.db                page     1
    80000008  READ         2  HELD        a.db                page     1
    80000006  WRITE        1  HELD        a.db                page     2
    80000008  WRITE        1  WAIT        a.db                page     2
    80000007  WRITE        1  HELD        a.db                page     3
    80000006  WRITE        1  WAIT        a.db                page     3

Now, each of the threads of control is blocked, waiting on a different thread of control. The thread using locker 80000007 is blocked by the thread using locker 80000008, due to the lock on page 4. The thread using locker 80000008 is blocked by the thread using locker 80000006, due to the lock on page 2. And the thread using locker 80000006 is blocked by the thread using locker 80000007, due to the lock on page 3. Since none of the threads of control can make progress, one of them will have to be killed in order to resolve the deadlock.

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