While a classical lock protects a memory area from explicit threads, the global lock of a synchronised block protects from all threads. That is the reason, why the following program is well-defined:. Although the variable i in line 7 is a global variable and the operations in the synchronized block are transaction-unsafe, the program is well-defined.
The access to i and std::cout happens in total order.
That is due to the synchronised block. The output of the program is not so thrilling. The values for i are written in an increasing sequence, separated by a comma. Only for completeness. What about data races? You can have them with synchronised blocks.
Software Transactional Memory in Java Using Multiverse
Only a small modification is necessary. To observe the data race, I let the synchronised block sleep for a nanosecond line At the same time, I access std::cout without using a synchronised block line 29 ; therefore I launch 10 threads that increment the global variable i. The output shows the issue. I put red circles around the issues in the output. These are the spots, at which std::cout is used by at least two threads at the same time.
But what is worse, is that the variable i is written by at least two threads. This is a data race. Therefore the program has undefined behaviour. If you look carefully at the output of the program you see that is written twice. You can execute transaction-unsafe code in a synchronised block but not in an atomic block. This holds true if the following properties do not apply to its definition. The next post is about the fork-join paradigm. To be specific, it's about task blocks. 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. Check that there is enough loo roll. If there is, do your business and wash your hands, of course. Exit the cubicle. Steal all the loo roll. Run away.
If the thief applies transactional lock elision, their procedure reduces to just a single instruction: T. If the toilet user does this, then the following unfortunate sequence of events can occur: The toilet user sees that the cubicle is vacant, but before entering , they check that there is enough loo roll.
While the toilet user is not looking for a moment, the thief sneaks in and steals all the loo roll. The toilet user enters and locks the cubicle, does their business, and discovers to their great surprise that there is no loo roll! Like this: Like Loading Leave a Reply Cancel reply Enter your comment here TR University of Rochester Computer Science Dept..
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Software Transactional Memory — PyPy documentation
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Chapter 28. Software transactional memory
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