deaddbg.so

berkeley db 4.6.21的源码。berkeley db是一个简单的数据库管理系统

m4_comment([$Id: deaddbg.so,v 10.5 2005/12/02 17:27:50 alanb Exp $])

m4_ref_title(Locking Subsystem,
    Deadlock debugging,, lock/timeout, lock/page)

m4_p([dnl
An occasional debugging problem in m4_db applications is unresolvable
deadlock.  The output of the m4_option(Co) flags of the m4_ref(db_stat)
utility can be used to detect and debug these problems.  The following
is a typical example of the output of this utility:])

m4_indent([dnl
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])

m4_p([dnl
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 m4_italic(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.)])

m4_p([dnl
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.])

m4_p([dnl
We also hold a write lock on the database page where we stored the new
key/data pair.  The page lock is labeled m4_italic(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:])

m4_indent([dnl
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])

m4_p([dnl
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:])

m4_indent([dnl
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])

m4_p([dnl
Here's a simple example of one lock blocking another one:])

m4_indent([dnl
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])

m4_p([dnl
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.])

m4_p([dnl
If lockers 80000004 and 80000005 are not in different threads of
control, the result would be m4_italic(self deadlock).  Self deadlock
is not a true deadlock, and won't be detected by the m4_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.])

m4_p([dnl
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.])

m4_indent([dnl
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])

m4_p([dnl
The three threads then each began a transaction, and put a key/data pair
on a different page:])

m4_indent([dnl
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])

m4_p([dnl
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.])

m4_p([dnl
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:])

m4_indent([dnl
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])

m4_p([dnl
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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