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

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Message-Id: <1235026040.8805.3.camel@pasglop>
Date:	Thu, 19 Feb 2009 17:47:20 +1100
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Jens Axboe <jens.axboe@...cle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: Q: smp.c && barriers (Was: [PATCH 1/4] generic-smp: remove
 single ipi fallback for smp_call_function_many())


> 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.


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