[LLVMdev] Multi-threading and mutexes in LLVM

[Zachary Turner]

+chandlerc, aaronballman, in case there are additional carryovers and/or
issues from the review thread which I've left out.


I have a patch up for review[1, 2] that attempts to replace LLVM's mutex
implementation with std::mutex and std::recursive_mutex.  While the patch
seems to work, there are questions surrounding whether or not the approach
used is correct.

I'll try to summarize the issues as best I can, in hopes of getting some
feedback from a broader audience, to make sure this is done correctly:

1) Should support multi-threading be a compile-time or runtime parameter in
LLVM?

Currently it is both.  It is compile-time through the use of the
define LLVM_ENABLE_THREADS, and it is runtime through the use of functions
llvm_start_multithreaded, llvm_is_multithreaded, etc.  I and some others
feel like runtime support for multi-threading could be removed, and it
should be compile-time only.  However, I am not aware of all the ways in
which this is being used, so this is where I would like some feedback.  The
issues I have with runtime multithreading support are the following:

* It leads to confusing code.  At any given point, is multi-threading
enabled or disabled?  You never know without calling llvm_is_multithreaded,
but even calling that is inherently racy, because someone else could
disable it after it returns.

* It leads to subtle bugs.  clang_createIndex, the first time it's called,
enables multi-threading.  What happens if someone else disables it later?
 Things like this shouldn't even be possible.

* Not all platforms even support threading to begin with.  This works now
because llvm_start_multithreaded(), if the compile time flag is set to
disable threads, simply does nothing.  But this decision should be made by
someone else.  Nobody really checks the return value from
llvm_start_multithreaded anyway, so there's already probably bugs where
someone tries to start multi-threading, and it fails.

* What does it actually mean to turn multi-threading support on and off?

Anybody that tries to do this is almost certainly broken due to some edge
cases about when it's on and when it's off.  So this goes back to the first
two points about confusing code and subtle bugs.

2) What should happen when you try to acquire a mutex in an app with
threading disabled?

If this is a compile-time parameter, the solution is simple: make an empty
mutex class that satisfies the Lockable concept, and have its methods do
nothing.  Then typedef something like llvm::mutex to be either std::mutex
or llvm::null_mutex accordingly.  If this is a runtime parameter, it's more
complicated, and we should ask ourselves whether or not it's necessary to
avoid the overhead of acquiring an uncontended mutex in single threaded
apps.  For what it's worth, all reasonable STL implementations will use a
lightweight mutex implementation, which generally require less than 100
nanoseconds to acquire uncontended, so I think we don't need to care, but
again some feedback would be nice.

3) We have some debug code in our mutex implementation that is intended to
try to help catch deadlocks and/or race conditions.  Do we need this code?

I think we can get by without it, but again some feedback about how, if at
all, people are using this would be nice.  For example, if you want to
detect deadlocks and race conditions you can use TSan.

Thanks!
Zach


[1] -
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140602/220051.html
[2] - http://reviews.llvm.org/D4033
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