How to Find which Session is Holding a Particular Library Cache Lock (Doc ID 122793.1)

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 holding 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:

$sqlplus '/ as sysdba'
oradebug setmypid
oradebug unlimit
oradebug dump systemstate 266

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

alter session set max_dump_file_size=unlimited;
alter session set events 'immediate trace name systemstate level 10'

Oracle will create a systemstate tracefile in your USER_DUMP_DEST directory.

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

select pid from v$process where addr=
(select paddr from v$session where sid= <sid_of_hanging_session> );

 

The systemstate dump contains a separate section with information for each process. 
Open the tracefile and do a search for "PROCESS <PID from above>".
In the process section, search for the wait event by doing a search on 'waiting for'.

PROCESS 20:
----------------------------------------
SO: 0x7d2bd820, type: 2, owner: (nil), flag: INIT/-/-/0x00
(process) Oracle pid=20, calls cur/top: 0x7d3d62d0/0x7d3d85dc, 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
(latch info) wait_event=0 bits=0
Process Group: DEFAULT, pseudo proc: 0x7d2ed5dc
O/S info: user: oracle, term: pts/7, ospid: 19759
OSD pid info: Unix process pid: 19759, image: xxxxxx.forgotten.realms (TNS V1-V3)

<cut>

(session) sid: 141 trans: (nil), creator: 0x7d2bd820, flag: (41) USR/- BSY/-/-/-/-/-
DID: 0001-0014-00000668, short-term DID: 0000-0000-00000000
txn branch: (nil)
oct: 6, prv: 0, sql: 0x62d01c34, psql: 0x7c20f24c, user: 542/<User Name>
service name: SYS$USERS
O/S info: user: oracle, term: pts/7, ospid: 19758, machine: xxxxxx.forgotten.realms
program: sqlplus@xxxxxx.forgotten.realms (TNS V1-V3)
application name: SQL*Plus, hash value=3669949024
waiting for 'library cache lock' blocking sess=0x(nil) seq=36 wait_time=0 seconds since wait started=11
handle address=62d064dc, lock address=79f88a68, 100*mode+namespace=c9

• 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=<User Name>.EMPLOYEES
  
  We see the library object lock is being requested in Shared mode (request=S)
  Name of the the object is <USER Name>.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=<User Name>.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:

select sid,saddr from v$session where event= 'library cache lock';

SID SADDR
---------- --------
16 572ed244


select kgllkhdl Handle,kgllkreq Request, kglnaobj Object
from x$kgllk where kgllkses = '572ed244'
and kgllkreq > 0;

HANDLE   REQUEST   OBJECT
-------- ---------- ------------------------------------------------------------
62d064dc          2 EMPLOYEES

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.

select kgllkses saddr,kgllkhdl handle,kgllkmod mod,kglnaobj object
from x$kgllk lock_a
where kgllkmod > 0
and exists (select lock_b.kgllkhdl from x$kgllk lock_b
where kgllkses = '572ed244' /* blocked session */
and lock_a.kgllkhdl = lock_b.kgllkhdl
and kgllkreq > 0);

SADDR     HANDLE   MOD
--------  -------- ----------
OBJECT
------------------------------------------------------------
572eac94  62d064dc          3
EMPLOYEES

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: 

select sid,username,terminal,program from v$session where saddr = '572eac94'

SID        USERNAME                  TERMINAL
---------- ------------------------------ ------------------------------
PROGRAM
------------------------------------------------
12          UserName                          pts/20
sqlplus@xxxxxx.forgotten.realms (TNS V1-V3)

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

select sid,username,terminal,program from v$session
where saddr in
(select kgllkses from x$kgllk lock_a
 where kgllkreq > 0
 and exists (select lock_b.kgllkhdl from x$kgllk lock_b
             where kgllkses = '572eac94' /* blocking session */
             and lock_a.kgllkhdl = lock_b.kgllkhdl
             and kgllkreq = 0)
);

SID        USERNAME                       TERMINAL               PROGRAM
-----------------------------------------------------------------------------------------------
13         UserName                           abc/22               sqlplus@go.there.com (TNS V1-V3)
16         UserName                           abc/7                sqlplus@go.there.com (TNS V1-V3)

REFERENCES

NOTE:1020008.6 - SCRIPT: FULLY DECODED LOCKING
NOTE:1054939.6 - How to: Determine if a Package that is About to be Compiled is Being Used Currently