Memory barriers and spin_unlock safety

David Howells dhowells at
Sat Mar 4 07:15:35 EST 2006

Linus Torvalds <torvalds at> wrote:

> The rules are, afaik, that reads can pass buffered writes, BUT WRITES 
> CANNOT PASS READS (aka "writes to memory are always carried out in program 
> order").

So in the example I gave, a read after the spin_unlock() may actually get
executed before the store in the spin_unlock(), but a read before the unlock
will not get executed after.

> No. Issuing a read barrier on one CPU will do absolutely _nothing_ on the 
> other CPU.

Well, I think you mean will guarantee absolutely _nothing_ on the other CPU for
the Linux kernel.  According to the IBM powerpc book I have, it does actually
do something on the other CPUs, though it doesn't say exactly what.

Anyway, thanks.

I'll write up some documentation on barriers for inclusion in the kernel.


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