The following list of instructions describes the procedure followed by a person who wishes to use the toilet. The following list of instructions describes the procedure followed by a person who is minded to steal loo roll. These two procedures have been carefully designed so that there can be no clashes between the toilet user and the loo roll thief. Either way, no awkward situations should ever arise. Let us now suppose that the loo roll thief uses a technique called transactional lock elision.
This is a technique invented by Ravi Rajwar and Jim Goodman in , and it is one of the main ways that transactions are used in practice. The idea is that by introducing a transaction, one can avoid the faff of actually locking and unlocking the cubicle door. If the thief applies transactional lock elision, their procedure reduces to just a single instruction:.
All is still fine: even though the thief does not actually lock or unlock the cubicle, there can still be no problems for the ordinary toilet user, because the thief only enters when the cubicle is vacant. However, let us now also suppose that the toilet user is permitted to carry out their instructions out-of-order just like an Arm processor. One of the reorderings that Arm processors are allowed to do is to swap instructions U2 and U3. If the toilet user does this, then the following unfortunate sequence of events can occur:.
In other words: current Arm processors are not wrong to allow this reordering. The regrettable situation only arises when you consider the combination of out-of-order execution and transactional memory, which is exactly what we have done in our work. A disastrous lack of loo roll — or, to return to the computing world: a failure of mutual exclusion — is a serious problem. If Arm is going to support transactional memory in its multiprocessors in the future, it must make changes to its rules about which instructions can execute out-of-order, so that cubicle doors must be locked without delay.
You are commenting using your WordPress. You are commenting using your Google account. You are commenting using your Twitter account. You are commenting using your Facebook account. Notify me of new comments via email. Notify me of new posts via email. Out-of-order execution is where a computer chooses, for performance reasons, to perform its instructions in an order different from that written by the programmer.
Usually out-of-order execution is invisible, but occasionally it can affect the outcome of multi-threaded programs. The entire transaction behaves as if it is a single, uninterruptible instruction.
What is the bug? Enter the cubicle and lock the door behind you. Scott Jul John's, NL, Canada. In conjunction with PODC' Please also download errata from www. Marathe and William N. Scott Oct Houston, TX. Hosking Transactional monitors for concurrent objects. In: ed. Volume of Lecture Notes in Computer Science. Published by Springer-Verlag. Yossi Lev and Mark Moir Fast read sharing mechanism for software transactional memory poster. Integrating support for undo with exception handling.
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Transactional Locking II.
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Maurice Herlihy and Eric Koskinen feb Torvald Riegel and Diogo B. Brum feb Michael F.
it.muculuna.ga Spear and Maged Michael and Michael L. Scott feb Inevitability Mechanisms for Software Transactional Memory. Unsal and Mateo Valero feb Hybrid Transactional Memory to accelerate safe lock-based transactions. Smith mar Coqa: Concurrent Objects with Quantized Atomicity. International Conference on Compiler Construction.
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