Aaron Klotz at Mozilla

My adventures as a member of Mozilla’s Platform Integration Team

Why I Prefer Using CRITICAL_SECTIONs for Mutexes in Windows Nightly Builds

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In the past I have argued that our Nightly builds, both debug and release, should use CRITICAL_SECTIONs (with full debug info) for our implementation of mozilla::Mutex. I’d like to illustrate some reasons why this is so useful.

They enable more utility in WinDbg extensions

Every time you initialize a CRITICAL_SECTION, Windows inserts the CS’s debug info into a process-wide linked list. This enables their discovery by the Windows debugging engine, and makes the !cs, !critsec, and !locks commands more useful.

They enable profiling of their initialization and acquisition

When the “Create user mode stack trace database” gflag is enabled, Windows records the call stack of the thread that called InitializeCriticalSection on that CS. Windows also records the call stack of the owning thread once it has acquired the CS. This can be very useful for debugging deadlocks.

They track their contention counts

Since every CS has been placed in a process-wide linked list, we may now ask the debugger to dump statistics about every live CS in the process. In particular, we can ask the debugger to output the contention counts for each CS in the process. After running a workload against Nightly, we may then take the contention output, sort it descendingly, and be able to determine which CRITICAL_SECTIONs are the most contended in the process.

We may then want to more closely inspect the hottest CSes to determine whether there is anything that we can do to reduce contention and all of the extra context switching that entails.

In Summary

When we use SRWLOCKs or initialize our CRITICAL_SECTIONs with the CRITICAL_SECTION_NO_DEBUG_INFO flag, we are denying ourselves access to this information. That’s fine on release builds, but on Nightly I think it is worth having around. While I realize that most Mozilla developers have not used this until now (otherwise I would not be writing this blog post), this rich debugger info is one of those things that you do not miss until you do not have it.

For further reading about critical section debug info, check out this archived article from MSDN Magazine.