Messages in this thread | | | Date | Thu, 22 May 2008 15:56:52 -0700 (PDT) | From | Trent Piepho <> | Subject | Re: [PATCH] [POWERPC] Improve (in|out)_beXX() asm code |
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On Fri, 23 May 2008, Benjamin Herrenschmidt wrote: > On Tue, 2008-05-20 at 15:53 -0700, David Miller wrote: >> From: Scott Wood <scottwood@freescale.com> >> Date: Tue, 20 May 2008 17:43:58 -0500 >>> David Miller wrote: >>>> The __volatile__ in the asm construct disallows movement of the >>>> inline asm relative to statements surrounding it. >>>> >>>> The only reason barrier() in kernel.h needs a memory clobber is >>>> because of a bug in ancient versions of gcc. In fact, I think >>>> that memory clobber might even be removable. >>> >>> Current versions of GCC seem quite happy to move non-asm memory accesses >>> around a volatile asm without a memory clobber; see the test Trent posted. >> >> Indeed, and even the GCC manual is clear about this. > > So what is the scope of that problem ? > > IE. Take an x86 version of that test, writing to memory, doing a writel > to some MMIO, then another memory write, can those be re-ordered with > the current x86 version of writel ?
Yes, the same thing can happen on x86. As far as I could tell, this is something that all other arches can have happen. Usually aliasing prevents it, but it's not hard to constuct a test case where it doesn't.
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