The Unabridged Pentium 4 IA32 Processor Genealogy

A Race Condition Can Present a Problem

Consider the following possibility:

1. The task executing on processor 0 reads the semaphore to determine the buffer's availability.

2. The task tests the semaphore's value and determines that the buffer is available (the semaphore value = zero).

3. Immediately after the task on processor 0 has completed the memory read to obtain and test the state of the semaphore value, a task executing on processor 1 has initiated a memory read request on the FSB to test the state of the same semaphore. It completes the read and begins testing the value.

4. The processor 0 task initiates a memory write on the FSB to update the semaphore to a non-zero value to mark the shared buffer as unavailable. After it completes the write, it considers ...