Debugging Transactions

The usual debugging operations such as breakpoints work normally within lock-based critical sections and from RCU read-side critical sections. However, in initial transactional-memory hardware implementations an exception within a transaction will abort that transaction, which in turn means that breakpoints abort all enclosing transactions.

So how can transactions be debugged?

1. Use software emulation techniques within transactions containing breakpoints. Of course, it might be necessary to emulate all transactions any time a breakpoint is set within the scope of any transaction. If the runtime system is unable to determine whether or not a given breakpoint is within the scope of a transaction, then it might be necessary to emulate all transactions just to be on the safe side. Of course, this approach might impose significant overhead, which might in turn obscure the bug being pursued.
2. Use only hardware TM implementations that are capable of handling breakpoint exceptions. Unfortunately, as of this writing (September 2008), all such implementations are strictly research prototypes.
3. Use only software TM implementations, which are (very roughly speaking) more tolerant of exceptions than are the simpler of the hardware TM implementations. Of course, software TM tends to have higher overhead than hardware TM, so this approach may not be acceptable in all situations.
4. Program more carefully, so as to avoid having bugs in the transactions in the first place. As soon as you figure out how to do this, please do let me know the secret!!!

There is some reason to believe that transactional memory will deliver productivity improvements compared to other synchronization mechanisms, but it does seem quite possible that these improvements could easily be lost if traditional debugging techniques cannot be applied to transactions. This seems especially true if transactional memory is to be used by novices on large transactions. In contrast, macho top-gun programmers might be able to dispense with such debugging aids, especially for small transactions.

Therefore, if transactional memory is to deliver on its productivity promises to novice programmers, the debugging problem does need to be solved.