Synchronization [was Re: The Magic Show: kernel_map() disappearing]

[Jesper Skov]

>>>>> "Cort" == Cort Dougan <cort at persephone.cs.nmt.edu> writes:

Cort> You're right, that was an oversight on my part.  I'll fix the
Cort> mistake.  I'll commit that to vger now.  I'm going to leave the
Cort> sync in there as well, to make sure that things stall until the
Cort> writes are done.

Cort> }The reordering thing can also be achieved with 'sync', but it
Cort> stalls }the CPU until all IO has completed. 'eieio' is the
Cort> correct solution }(IMHO.. And Motos NSHO btw)


OK, core of the problem is that I don't know what mb() is supposed to
do. Here's what we have on the PPC:

 sync : stall CPU until all IO has completed.
 eieio: prevent CPU from reordering/collapsing reads/writes to memory.
        (but otherwise run at full steam!)

We would want to use eieio for drivers and such accessing their
controller in a strict serialized manner. We would want to use sync
for SMP related stuff since it ensures, well, synchronization between
multiple CPUs wrt memory access.

Using sync for drivers is a bad idea; it's overkill, stalling the
CPU for no good reason.

That's why I added the iobarrier() macros (adding _w/_r/_rw on Alan's
request) that are only related to IO synchronization within the same
CPU. Essentially it's just renaming the eieio() macro already used in
many PPC drivers because it's a bad name; Linux/APUS will be sharing
drivers with Linux/m68k where eieio isn't such a helpful name.

Given that mb() was defined as sync, I assumed mb() to be the common
Linux name for a thingie used for SMP synchronization.

I may have been wrong. iobarrier() may not be such a good name after
all, but I think it's important for the names to reflect the semantics
of the function. And there are two separate synchronization semantics
required (at least on the PPC) - I don't want to merge them just
because there's already a common Linux function that's named mb()
(memory barrier, presumably).

Don't know what the functions/macros should be named, but it would be
nice if:

 a) the names reflected semantics: CPU internal IO synchronization vs
    inter-CPU synchronization.

 b) the names were common on all Linux
    archs, defined as do {} while(0) when not applicable.


For bigger brains to discuss further. I rest my case :)

Jesper

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