Linux内核内存障碍

Question

I am new to Linux kernel programming. I am confused by the memory barriers documentation, in the GUARANTEES chapter.

Overlapping loads and stores within a particular CPU will appear to be ordered within that CPU.

 *X = c; d = *X; 

the CPU will issue:

 STORE *X = c, d = LOAD *X 

And then another example after that:

It must be assumed that overlapping memory accesses may be merged or discarded.

 *A = X; Y = *A; 

we may get

 STORE *A = X; Y = LOAD *A; STORE *A = Y = X; 

They look like the same, what's the difference between the two examples?

Answer 1

You are right, the documnetation is not correct. Actually your version of the file isn't up to date. There have been a commit in 3.8-rc3 which fixed the issue: http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=f191eec58803e1e16c3421638cdcc9195c425851

Hope that helps. Matthias