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DR database crashed with below errors..

Client address: (ADDRESS=(PROTOCOL=<protocol>)(HOST=<hostname>)(PORT=<port>))
WARNING: inbound connection timed out (ORA-3136)
Wed Jul 13 13:43:24 2016
Errors in file /<path>/diag/rdbms/<db_name>/<oracle_sid>/trace/<oracle_sid>_lgwr_<pid>.trc:
ORA-04021: timeout occurred while waiting to lock object
LGWR (ospid: 31312): terminating the instance due to error 4021
Wed Jul 13 13:43:24 2016
System state dump requested by (instance=1, osid=31312 (LGWR)), summary=[abnormal instance termination].
System State dumped to trace file /<path>/diag/rdbms/<db_name>/<oracle_sid>/trace/<oracle_sid>_diag_<pid>.trc
Wed Jul 13 13:43:25 2016
License high water mark = 318
Instance terminated by LGWR, pid = 31312
USER (ospid: 20898): terminating the instance
Instance terminated by USER, pid = 20898
Wed Jul 13 13:43:39 2016
Starting ORACLE instance (normal)

Bug 16717701 - ADG SHOULD GET THE INSTANCE PARSE LOCK WITH A TIMEOUT  ------> Superseded By Bug fix Bug 17018214

Bug 11712267 - ACTIVE DATA GUARD DATABASE HUNG ON 'LIBRARY CACHE: MUTEX X' WAIT EVENT

LGWR trace file (RXEPRR1_lgwr_31312.trc)

*** 2016-07-13 13:43:24.498
*** SESSION ID:(6709.1) 2016-07-13 13:43:24.498
*** CLIENT ID:() 2016-07-13 13:43:24.498
*** SERVICE NAME:(SYS$BACKGROUND) 2016-07-13 13:43:24.498
*** MODULE NAME:() 2016-07-13 13:43:24.498
*** ACTION NAME:() 2016-07-13 13:43:24.498

error 4021 detected in background process
ORA-04021: timeout occurred while waiting to lock object
kjzduptcctx: Notifying DIAG for crash event
----- Abridged Call Stack Trace -----
ksedsts()+1296<-kjzdicrshnfy()+364<-ksuitm()+1688<-ksbrdp()+4296<-opirip()+1680<-opidrv()+748<-sou2o()+88<-opimai_real()+276<-ssthrdmain()+316<-main()+316<-_start()+380
----- End of Abridged Call Stack Trace -----

Issue matches with bug 11712267 and bug 16717701

Since two bugs are matching with the case,

You can try with option (1) . As per Bug 11712267

change the cursor_sharing to force on Active dataguard (ADG).

Monitor your environment for sometime.

If it crashes again then follow with the option (2)
Option (2):

As per bug description

LGWR can request DBINSTANCE lock in X mode without any timeout which can lead to a hang / deadlock.

Both fixes are already included in 11.2.0.4 but the fix is DISABLED by default.
== > To ENABLE the fix one has to set == > "_adg_parselock_timeout" > to the number of centi-seconds == > LGWR should wait before backing off and retrying the request.

Value should be in centi seconds. == > I Don't think there is really any hard fast rule for a value - at default (0) it will not timeout.
A value representing a few seconds seems reasonable - if LGWR has been stuck for say 5 seconds waiting it seems reasonable guess it is not going to get the lock.

The param just causes it to abort the current attempt and retry If you want to play safe can start with a higher value then decrease later.
A higher value will just mean more sessions blocked for longer in case of the deadlock situation.
500 Seems reasonable , but I have no data to base it on.

There should be a statistic "ADG parselock X get attempts" If it gets set too small that value would likely increase a lot due to keep timing out and retrying.

This is a dynamic parameter

Follow option (1) .

change the cursor_sharing to force on ADG

If issue re-appears then follow option (2) as below

Please set "_adg_parselock_timeout" to 500 == >

SQL > alter system set "_adg_parselock_timeout"=500 scope=both sid='*';