NTDebuggers, one of the most important things to understand in kernel debugging hung servers is the output of !locks. There can be a lot of data and it’s not always clear what is going on. One of the things I like to do in order to better understand the output is to use a visual representation of the resources involved and the threads that are blocking on those resources. Before we can do that we need to understand what to look for so we can document it in our diagram.
It’s a good idea to understand ERESOURCEs in general l before jumping into !locks. The following MSDN article goes into lots of great detail. http://msdn2.microsoft.com/en-us/library/aa490224.aspx
Simply put, you will typically see threads either with access to or trying to gain access to resources. If a thread has access to a resource it will be marked by <*>. Threads that have access to a resource can block other threads from gaining access to said resource.
You will see threads waiting for shared access. These threads do not have the <*> and listed above the threads that are Waiting on Exclusive Access.
You will also see threads that are Waiting on Exclusive Access. These threads are typically blocked waiting for the threads that have access or ownership of the resource to release it.
Let’s take a look at one section of !locks output and annotate each thread section...
Resource @ 0x896d2a68 Shared 1 owning threads << This info is the ERESOURCE in question. Contention Count = 15292 << The amount of contention for the object.
NumberOfSharedWaiters = 1 << This is self explanatory
NumberOfExclusiveWaiters = 39 << Number of exclusive waiters in the Ex Waiter List
Threads: 89bd1234-01<*> 896d2020-01 << We have two threads here. The owner, or shared owner <*>89bd1234 and the shared Waiter 896d2020
Threads Waiting On Exclusive Access:
888ed020 87c036f8 885dc7a0 8bc538b0 << All of these threads are waiting on exclusive access.
88e8cda0 88796988 8905fda0 8974dc10
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