How to Find which Session is Holding a Particular Library Cache Lock

APPLIES TO:

PURPOSE

Purpose

In some situations it may happen that your session is hanging and is waiting for a 'Library cache lock'. This document describes how to find the session that is holdig the lock that you are waiting for.

 Scope and Application

Support Analysts and DBAs

TROUBLESHOOTING STEPS

Common Situations 

• A DML operation that is hanging because the table which is accessed is currently undergoing changes (ALTER TABLE). This may take quite a long time depending on the size of the table and the type of the modification (e.g. ALTER TABLE x MODIFY (col1 CHAR(200) on a table with thousands of records)

  In this case, V$LOCK will show that the session doing the 'ALTER TABLE' with an exclusive DML enqueue lock on the table object (LMODE=6, TYPE=TM where ID1 is the OBJECT_ID of the table). The waiting session however does not show up in V$LOCK. So in an environment with a lot of concurrent sessions, the V$LOCK information will be insufficient to track down the culprit blocking your operation.

• The compilation of package will hang on Library Cache Lock and Library Cache Pin if any users are executing a procedure/function defined in the same package.

Method 1: Systemstate Analysis

Systemstate event will create a tracefile containing detailed information on every Oracle process. This information includes all the resources held & requested by a specific process.

While an operation is hanging, open a new session and launch the following statement:

For Oracle 9.2.0.1 or higher:

For older versions, you can use the following syntax that is also possible in higher versions.The level 266 is not available before 9.2.0.6

Oracle will create a systemstate tracefile in your USER_DUMP_DEST directory.

Get the PID (ProcessID) of the 'hanging' session:

• Use the handle address to find information on the object locked:
  SO: 0x79f88a68, type: 53, owner: 0x7d3d62d0, flag: INIT/-/-/0x00
  LIBRARY OBJECT LOCK: lock=79f88a68 handle=62d064dc request=S
  call pin=(nil) session pin=(nil) hpc=0000 hlc=0000
  htl=0x79f88ab4[0x79e71e60,0x79e71e60] htb=0x79e71e60 ssga=0x79e716fc
  user=7d3a13b8 session=7d3a13b8 count=0 flags=[0000] savepoint=0xce
  LIBRARY OBJECT HANDLE: handle=62d064dc mtx=0x62d06590(0) cdp=0
  name=SCOTT.EMPLOYEES
  
  We see the library object lock is being requested in Shared mode (request=S)
  Name of the the object is SCOTT.EMPLOYEES
  
  
• Use the 'handle address' to find the process that is holding the lock on your resource by doing a search on the address within the same tracefile.
  PROCESS 18: 
  ---------------------------------------- 
  SO: 0x7d2bcca8, type: 2, owner: (nil), flag: INIT/-/-/0x00 
  (process) Oracle pid=18, calls cur/top: 0x79f3ab84/0x7d3d5fc8, flag: (0) - 
  int error: 0, call error: 0, sess error: 0, txn error 0 
  (post info) last post received: 109 0 4 
  last post received-location: kslpsr 
  last process to post me: 7d2b8d94 1 6 
  last post sent: 0 0 24 
  last post sent-location: ksasnd 
  last process posted by me: 7d2b8d94 1 6 
  
  <cut> 
  
  SO: 0x75fe8f7c, type: 53, owner: 0x7b751914, flag: INIT/-/-/0x00 
  LIBRARY OBJECT LOCK: lock=75fe8f7c handle=62d064dc mode=X 
  call pin=(nil) session pin=(nil) hpc=0000 hlc=0000 
  htl=0x75fe8fc8[0x79f81790,0x79fc3ef8] htb=0x79f81790 ssga=0x79f8102c 
  user=7d3988d0 session=7d3988d0 count=1 flags=[0000] savepoint=0x146e 
  LIBRARY OBJECT HANDLE: handle=62d064dc mtx=0x62d06590(0) cdp=0 
  name=SCOTT.EMPLOYEES
  
  From the output we can see that the Process 18 (pid) is holding an exclusive lock (mode=X) on the object we are trying to access. Using V$PROCESS and V$SESSION, we can retrieve the sid, user, terminal, program,... for this process.
  The actual statement that was launched by this session is also listed in the tracefile (statements and other library cache objects are preceded by 'name=').

METHOD 2: EXAMINE THE X$KGLLK TABLE

The X$KGLLK table (accessible only as SYS/INTERNAL) contains all the library object locks (both held & requested) for all sessions and is more complete than the V$LOCK view although the column names don't always reveal their meaning.

You can examine the locks requested (and held) by the waiting session by looking up the session address (SADDR) in V$SESSION and doing the following select:

This will show you the library cache lock requested by this session (KGLLKREQ > 0) where KGLNAOBJ contains the first 80 characters of the name of the object.The value in KGLLKHDL corresponds with the 'handle address' of the object in Method 1 Systemstate Analysis as shown above.

If we now match the KGLLKHDL with the handles of other sessions in X$KGLLK, that should give us the address of the blocking session.The session holding the lock will have KGLLKMOD > 0 as it is holding the lock.

If we look a bit further, we can then again match KGLLKSES with SADDR in v$session to find further information on the blocking session: 

In the same way, we can also find all the blocked sessions: