Suppose, on a multiprocessor machine, there are two global variables A and B, each one byte in size, located near each other in memory, and two CPUs executing the following code.
CPU 1:
read A
calculate new value
write A
CPU 2:
read B
calculate new value
write B
Just looking at what would tend to physically happen, we would expect the above would be incorrect without any explicit locking because A and B could be in the same cache line, and CPU 1 needs to read the entire cache line, change the value of a single byte and write the line again; if CPU 2 does its read-modify-write of the cache line in between, the update to B could be lost. (I'm assuming it doesn't matter what order A and B are updated in, I'm only concerned with making sure neither update is lost.)
But x86 guarantees this code is okay. On x86, a write to a single variable only becomes non-atomic if that variable is misaligned or bigger than the CPU word size.
Does an x86 CPU automatically carry out extra locking on the front side bus in order to make such individual variable updates, work correctly without explicit locking?