前几天,一台Oracle数据库（Oracle Database 10g Release 10.2.0.4.0 - 64bit Production）监控出现"PMON failed to acquire latch, see PMON dump"错误，连接数据库出现短暂异常，告警日志中具体错误如下所示：
Tue Dec 20 09:13:16 2016
PMON failed to acquire latch, see PMON dump
Tue Dec 20 09:14:16 2016
PMON failed to acquire latch, see PMON dump
Tue Dec 20 09:15:55 2016
PMON failed to acquire latch, see PMON dump
Tue Dec 20 09:17:15 2016
PMON failed to acquire latch, see PMON dump
Tue Dec 20 09:17:24 2016
WARNING: inbound connection timed out (ORA-3136)
Tue Dec 20 09:18:23 2016
PMON failed to acquire latch, see PMON dump
Tue Dec 20 09:19:24 2016
PMON failed to acquire latch, see PMON dump
在生成的epps_pmon_4988.trc 跟踪文件里面，发现有些详细信息，你会发现PMON进程不能获取‘Child shared pool‘ latch，它被一个pid = 19 ospid=5022的进程给阻塞了。而ospid这个进程是一个Dispatcher的进程。
*** 2016-12-20 09:14:16.575
PMON unable to acquire latch 600edfa8 Child shared pool level=7 child#=1
Location from where latch is held: kghfrunp: alloc: session dur:
Context saved from call: 0
state=busy, wlstate=free
waiters [orapid (seconds since: put on list, posted, alive check)]:
33 (3, 1482196555, 3)
10 (3, 1482196555, 3)
25 (3, 1482196555, 3)
13 (3, 1482196555, 3)
waiter count=4
gotten 861091119 times wait, failed first 7114074 sleeps 1392223
gotten 0 times nowait, failed: 0
possible holder pid = 19 ospid=5022
----------------------------------------
SO: 0x40979aec8, type: 2, owner: (nil), flag: INIT/-/-/0x00
(process) Oracle pid=19, calls cur/top: (nil)/0x409c92608, flag: (80) DISPATCHER
int error: 0, call error: 0, sess error: 0, txn error 0
(post info) last post received: 0 0 236
last post received-location: kmcpdp
last process to post me: 4097a64a0 106 64
last post sent: 0 0 229
last post sent-location: kmcmbf: not KMCVCFTOS
last process posted by me: 4097a64a0 106 64
(latch info) wait_event=0 bits=a0
holding (efd=4) 600edfa8 Child shared pool level=7 child#=1
Location from where latch is held: kghfrunp: alloc: session dur:
Context saved from call: 0
state=busy, wlstate=free
waiters [orapid (seconds since: put on list, posted, alive check)]:
33 (3, 1482196555, 3)
10 (3, 1482196555, 3)
25 (3, 1482196555, 3)
13 (3, 1482196555, 3)
waiter count=4
holding (efd=4) 3fff78210 Child library cache level=5 child#=2
Location from where latch is held: kghfrunp: clatch: wait:
Context saved from call: 0
state=busy, wlstate=free
waiters [orapid (seconds since: put on list, posted, alive check)]:
15 (3, 1482196555, 3)
17 (3, 1482196555, 3)
12 (3, 1482196555, 3)
waiter count=3
Process Group: DEFAULT, pseudo proc: 0x4098bc190
O/S info: user: oracle, term: UNKNOWN, ospid: 5022
OSD pid info: Unix process pid: 5022, image: [email protected] (D007)
Short stack dump:
ksdxfstk()+32<-ksdxcb()+1547<-sspuser()+111<-__restore_rt()+0<-kghfrunp()+1506<-kghfnd()+1389<-kghalo()+587<-kmnsbm()+578<-nsb
al()+428<-nsbalc()+123<-nsdo()+17278<-nsopen()+2315<-nsanswer()+512<-kmnans()+37<-kmdahd()+385<-kmdmai()+5220<-kmmrdp()+564<-o
pirip()+1193<-opidrv()+582<-sou2o()+114<-opimai_real()+317<-main()+116<-__libc_start_main()+244<-_start()+41
Dump of memory from 0x0000000409747C68 to 0x0000000409747E70
409747C60 00000001 00000000 [........]
409747C70 FE9BEE10 00000003 0000003A 0003129B [........:.......]
409747C80 FEA7D5D0 00000003 0000003A 0003129B [........:.......]
409747C90 FE9DAD30 00000003 0000003A 0003129B [0.......:.......]
Repeat 2 times
409747CC0 FEAB01F0 00000003 0000003A 0003129B [........:.......]
409747CD0 FE9DAD30 00000003 0000003A 0003129B [0.......:.......]
409747CE0 FEA44E70 00000003 0000003A 0003129B [pN......:.......]
409747CF0 FEAA6FF0 00000003 0000003A 0003129B [.o......:.......]
409747D00 FEAB8AD0 00000003 0000003A 0003129B [........:.......]
409747D10 FEA14FF0 00000003 0000003A 0003129B [.O......:.......]
409747D20 FE9A77F0 00000003 0000003A 0003129B [.w......:.......]
Repeat 1 times
409747D40 FEA3CEB0 00000003 0000003A 0003129B [........:.......]
Repeat 1 times
409747D60 FE9C64B0 00000003 0000003A 0003129B [.d......:.......]
Repeat 1 times
409747D80 FEA062B0 00000003 0000003A 0003129B [.b......:.......]
Repeat 3 times
409747DC0 FEAA6FF0 00000003 0000003A 0003129B [.o......:.......]
409747DD0 FEA8F9D0 00000003 0000003A 0003129B [........:.......]
409747DE0 FE9F7570 00000003 0000003A 0003129B [pu......:.......]
409747DF0 FEA91530 00000003 0000003A 0003129B [0.......:.......]
409747E00 FE9BEE10 00000003 0000003A 0003129B [........:.......]
409747E10 FE9BB750 00000003 0000003A 0003129B [P.......:.......]
409747E20 FEA90C10 00000003 0000003A 0003129B [........:.......]
409747E30 FEA8B9F0 00000003 0000003A 0003129B [........:.......]
409747E40 FE9C5270 00000003 0000003A 0003129B [pR......:.......]
409747E50 FEAE12B0 00000003 0000003A 0003129B [........:.......]
409747E60 FE9C5270 00000003 0000003A 0003129B [pR......:.......]
由于当时没有出现问题时，并没有及时发现，没有Collect HangAnalyze traces，所以再深入一点的挖掘root case已经很难了。当时手工生成了一个快照（9:26），也就是说9:00 ~ 9:26这段时间生成的快照刚刚覆盖了出现问题的时间段。生成了这个时段的AWR报告，在这个时间段latch:library cache 和latch：shared pool等待事件是主要等待事件。
出现问题的时间段，数据库服务器是比较空闲的。
生成了20-Dec-16 09:11:16到20-Dec-16 09:21:16时段的ASH报告。如下所示，latch:library cache 和latch：shared pool为主要等待事件，但是Avg Active Sessions很小。
所以觉得很有可能是跟Bug有关系，后面在Oracle MetaLink查了一下是否有相关Bug，如下一些相关资料：
Bug 7039896 Spin under kghquiesce_regular_extent holding shared pool latch with AMM
Bug 6488694 - DATABASE HUNG WITH PMON FAILED TO ACQUIRE LATCH MESSAGE
Note 7039896.8 - Bug 7039896 - Spin under kghquiesce_regular_extent holding shared pool latch with AMM
Pmon Failed To Acquire Latch" Messages in Alert Log -Database Hung (文档 ID 468740.1)
Hang (Involving Shared Resource)
A process may hold a shared resource a lot longer than normally expected leading to many other processes having to wait for that resource. Such a resource could be a lock, a library cache pin, a latch etc.. The overall symptom is that typically numerous processes all appear to be stuck, although some processes may continue unhindered if they do not need the blocked resource.
Hang (Process Spins)
A process enters a tight CPU loop so appears to hang but is actually consuming CPU.
Latch Contention
This issue can result in latch contention within the database.
Waits for "latch: shared pool"
我们数据库版本为Oracle Database 10g Release 10.2.0.4.0 - 64bit Production， 所以Bug 7039896是会影响的这个数据库的， 而出现的现象也很符合，但是有一点就是并没有涉及MMAN进程。而且查过V$SGA_RESIZE_OPS，那个时间段并没有相关组件的增长、收缩。另外跟Bug 也非常类似，但是trc文件并没有发现跟MMAN进程有关系。 这个问题还是第一次出现，而且出现过一次后，最近几天都没有出现，所以更加确信是Bug引起的。当然是要找个时间应用Bug 7039896的相关补丁。
另外，在查找这个问题的时候，在官方文档看到一个如何处理、诊断‘PMON failed to acquire latch, see PMON dump‘的详细文档，本想收录于此，不过还是保持为PDF文件较好，需要可从下面链接下载。