问题描述：
现网个别时候会出现CPU突然飙高的现象，飙高后不能恢复正常。
分析过程：
CPU飙高后抓dump，最好本机看，其它机器看dump可能需要下载服务运行机器的sos，clr
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0:000> !threadpool
The version of SOS does not match the version of CLR you are debugging.??Please
load the matching version of SOS for the version of CLR you are debugging.
CLR Version: 4.0.30319.276
SOS Version: 4.0.30319.17929
CPU utilization: 100%
Worker Thread: Total: 54 Running: 54 Idle: 0 MaxLimit: 32767 MinLimit: 8
Work Request in Queue: 0
--------------------------------------
Number of Timers: 3
--------------------------------------
Completion Port Thread:Total: 10 Free: 10 MaxFree: 16 CurrentLimit: 8 MaxLimit: 1000 MinLimit: 8
CPU确实高，54个线程都在跑着。
接着看线程都在干什么
0:000> ~*e!clrstack
OS Thread Id: 0x3278 (57)
Child SP?????????IP???????????????Call Site
000000000c72cea8 0000000077ef047a [RedirectedThreadFrame: 000000000c72cea8]
000000000c72cf40 000006448056c07e?********************.
SecurityQuota.ConcurrentLinkedQueue`1[[System.__Canon, mscorlib]].TryDequeue(System.__Canon ByRef)
000000000c72cff0 00000644805999e8?********************.SecurityQuota.SecurityFeatureDigest.GetIncrementalDigest(********************)
……
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有50个线程在做该操作，review该处代码
为了防止本地代码和服务器不一致，直接反编译的服务器的代码，GetIncrementalDigest方法如下
public class SecurityFeatureDigest : ISecurityFeatureDigest
{
????// Fields
????private static Database _data = DatabaseManager.GetDatabase("SQSDB");
????private static readonly ITracing _tracer = TracingManager.GetTracing(typeof(SecurityFeatureDigest));
????private static ConcurrentLinkedQueue<List<FeatureDigest>> queue =
new ConcurrentLinkedQueue<List<FeatureDigest>>(new List<FeatureDigest>());?????(2)
????private static readonly string[] SetFeatureDigestParameter = new string[] { "@digest", "@operation", "@sample", "@stamp" };
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????// 略…
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????public void GetIncrementalDigest(RpcServerContext context)
????{
????????DateTime args = context.GetArgs<DateTime>();
????????List<FeatureDigest> results = new List<FeatureDigest>();
????????List<FeatureDigest> result = null;
????????while (true)
????????{
????????????if (queue.TryDequeue(out result))??????(1)
????????????{
????????????????List<FeatureDigest> list3 = new List<FeatureDigest>();
????????????????list3.AddRange(result);
????????????????queue.Enqueue(result);
????????????????foreach (FeatureDigest digest in list3)
????????????????{
????????????????????if (digest.Version > args)
????????????????????{
????????????????????????digest.Sample = string.Empty;
????????????????????????results.Add(digest);
????????????????????}
????????????????}
????????????????context.Return<List<FeatureDigest>>(results);
????????????????return;
????????????}
????????}
????}
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????// 略…
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}
从抓的dump看，极有可能是死循环了。通过以上代码看到，如果(1)处的返回为false的话，则改段代码会陷入死循环。接着看下什么情况下返回false。
public class ConcurrentLinkedQueue<T>
{
????// Fields
????private Node<T, T> _head;
????private Node<T, T> _tail;
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????// 略…
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????public bool TryDequeue(out T result)
????{
????????while (true)
????????{
????????????Node<T, T> comparand = this._head;
????????????Node<T, T> node2 = this._tail;
????????????Node<T, T> next = comparand.Next;
????????????if (comparand == this._head)
????????????{
????????????????if (next == null)???????(3)
????????????????{
????????????????????result = default(T);
????????????????????return false;
????????????????}
????????????????if (comparand == node2)
????????????????{
????????????????????Interlocked.CompareExchange<Node<T, T>>(ref this._tail, next, node2);
????????????????}
????????????????else
????????????????{
????????????????????result = next.Item;
????????????????????if (Interlocked.CompareExchange<Node<T, T>>(ref this._head, next, comparand) == comparand)
????????????????????{
????????????????????????break;
????????????????????}
????????????????}
????????????}
????????}
????????return true;
????}
从(3)处可以看出，如果head的next为null的话，则返回false。为了确认分析是否正确，看下此处的值是不是null
0:000> !dumpheap -stat
Statistics:
??????????????MT????Count????TotalSize Class Name
000006448054b2d0????????1???????????32?********************.SecurityQuota.ConcurrentLinkedQueue`1
[[System.Collections.Generic.List`1[[********************.FeatureDigest, IICSecurityLibrary]], mscorlib]]
0:000> !dumpheap – -mt 000006448054b2d0???
------------------------------
Heap 6
?????????Address???????????????MT?????Size
00000001404836f0?000006448054b2d0???????32????
total 0 objects
Statistics:
??????????????MT????Count????TotalSize Class Name
000006448054b2d0????????1???????????32?********************.SecurityQuota.ConcurrentLinkedQueue`1
[[System.Collections.Generic.List`1[[********************.FeatureDigest,?*****]], mscorlib]]
Total 1 objects
0:000> !do 00000001404836f0
Name:????????********************.SecurityQuota.ConcurrentLinkedQueue`1[[System.Collections.Generic.List`1
[[********************.FeatureDigest,?*****]], mscorlib]]
MethodTable: 000006448054b2d0
EEClass:?????0000064480557a90
Size:????????32(0x20) bytes
File:????????********************\SecurityQuotaService.exe
Fields:
??????????????MT????Field???Offset?????????????????Type VT?????Attr????????????Value Name
000006448054b4d0??4000001????????8 ...Canon, mscorlib]]??0 instance?00000000c3135ec8?_head
000006448054b4d0??4000002???????10 ...Canon, mscorlib]]??0 instance?00000000c3135ec8?_tail
0:000> !do 00000000c3135ec8
Name:????????********************.SecurityQuota.ConcurrentLinkedQueue`1+Node`1[[System.Collections.Generic.List`1[[********************.
FeatureDigest,?*****]], mscorlib],[System.Collections.Generic.List`1[[********************.FeatureDigest,?*****]], mscorlib]]
MethodTable: 000006448054b7f8
EEClass:?????00000644805580b8
Size:????????32(0x20) bytes
File:????????********************\SecurityQuotaService.exe
Fields:
??????????????MT????Field???Offset?????????????????Type VT?????Attr????????????Value Name
00000644784c6040??4000003????????8???????System.__Canon??0 instance 00000001404836a8 Item
000006448054b4d0??4000004???????10 ...Canon, mscorlib]]??0 instance?0000000000000000?Next
发现Next为null
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问题结论：
由于一些异常逻辑导致程序死循环，此处代码也有问题，对异常逻辑处理不够。
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作者:No.40
Blog:http://www.cnblogs.com/no40
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