While debugging with the SOS extension, I used the SyncBlk command and got
the following output.
0:003> !SyncBlk
Index SyncBlock MonitorHeld Recursion Thread ThreadID Object Waiting
3 0016f634 3 1 161a50 910 4 00aa1bac
System.Object
Waiting threads:
4 0016f660 3 1 161888 d08 3 00aa1ba0
System.Object
Waiting threads:
I was curious to know what that MonitorHeld column was - and dug through the
Rotor sos code and found that it mapped to the m_MonitorHeld of AwareLock.
The lines of code that interest me are (in SyncBlk.cpp)
FastInterlockCompareExchange((LONG*)&m_MonitorHeld, state + 2, state) (which
gets executed whenever tries to acquire a lock)
And
FastInterlockCompareExchange((LONG*)&m_MonitorHeld, state - 2, state)
(inside AwareLock::EnterEpilog)
Whenever you get a new guy waiting on the lock, the state variable (which is
the same as the m_MonitorHeld) gets bumped up by 2 and bumped back down by 2
whenever someone stops waiting.
When someone *does* acquire the lock, the code executed is
(FastInterlockCompareExchange((LONG*)&m_MonitorHeld, ((state - 2) | 1),
state).
Now, here are my questions
1. Why is the number bumped up by 2 every time someone waits on the lock?
2. What is the meaning of doing (state - 2) | 1) - and why?
3. How do I translate the MonitorHeld column in the !SyncBlk command? Do I
assume that every 2 incrememnt after 1 corresponds to some guy waiting?
Thanks in advance,
Sriram
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I blog at http://www.dotnetjunkies.com/weblog/sriram
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