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

[Linus Torvalds]

On Wed, 18 Feb 2009, Nick Piggin wrote:
> 
> I agree with you both that we *should* make arch interrupt code
> do the ordering, but given the subtle lockups on some architectures
> in this new code, I didn't want to make it significantly weaker...
> 
> Though perhaps it appears that I have, if I have removed an smp_mb
> that x86 was relying on to emit an mfence to serialise the apic.

The thing is, if the architecture doesn't order IPI wrt cache coherency, 
then the "smp_mb()" doesn't really do so _either_. 

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.

			Linus


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