Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove single ipi fallback for smp_call_function_many())

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

> It might hide some architecture-specific implementation issue, of course, 
> so random amounts of "smp_mb()"s sprinkled around might well make some 
> architecture "work", but it's in no way guaranteed. A smp_mb() does not 
> guarantee that some separate IPI network is ordered - that may well take 
> some random machine-specific IO cycle.
> 
> That said, at least on x86, taking an interrupt should be a serializing 
> event, so there should be no reason for anything on the receiving side. 
> The _sending_ side might need to make sure that there is serialization 
> when generating the IPI (so that the IPI cannot happen while the writes 
> are still in some per-CPU write buffer and haven't become part of the 
> cache coherency domain).
> 
> And at least on x86 it's actually pretty hard to generate out-of-order 
> accesses to begin with (_regardless_ of any issues external to the CPU). 
> You have to work at it, and use a WC memory area, and I'm pretty sure we 
> use UC for the apic accesses.

On powerpc, I suspect an smp_mb() on the sender would be useful... it
mostly depends how the IPI is generated but in most case it's going to
be an MMIO write, ie non-cached write which isn't ordered vs. any
previous cached store except using a full sync (which is what smp_mb()
does).

Cheers,
Ben.