Messages in this thread | | | Date | Wed, 17 Oct 2007 02:30:32 +0200 (CEST) | From | Mikulas Patocka <> | Subject | Re: LFENCE instruction (was: [rfc][patch 3/3] x86: optimise barriers) |
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> > You already must not place any data structures into WC memory --- for > > example, spinlocks wouldn't work there. > > What do you mean "already"?
I mean "in current kernel" (I checked it in 2.6.22)
> If we already have drivers loading data from > WC memory, then rmb() needs to order them, whether or not they actually > need it. If that were prohibitively costly, then we'd introduce a new > barrier which does not order WC memory, right? > > > > wmb() also won't work on WC > > memory, because it assumes that writes are ordered. > > You mean the one defined like this: > #define wmb() asm volatile("sfence" ::: "memory") > ? If it assumed writes are ordered, then it would just be a barrier().
You read wrong part of the include file. Really, it is (2.6.22,include/asm-i386/system.h): #ifdef CONFIG_X86_OOSTORE #define wmb() alternative("lock; addl $0,0(%%esp)", "sfence", X86_FEATURE_XMM) #else #define wmb() __asm__ __volatile__ ("": : :"memory") #endif
CONFIG_X86_OOSTORE is dependent on MWINCHIP3D || MWINCHIP2 || MWINCHIPC6 --- so on Intel and AMD, it is really just barrier().
So drivers can't assume that wmb() works on write-combining memory.
> > > Doing that would lead to an unmaintainable mess. If drivers don't > > > need rmb, then they don't call it. > > > > If wmb() doesn't currently work on write-combining memory, why should > > rmb() work there? > > I don't understand why you say wmb() doesn't work on WC memory.
Because it is defined as __asm__ __volatile__ ("": : :"memory")
And WC memory can reorder writes (WB memory can't).
> > The purpose of rmb() is to enforce ordering on architectures that don't > > force it in hardware --- that is not the case of x86. > > Well it clearly is the case because I just pointed you to a document > that says they can go out of order.
> If you want to argue that existing > implementations do not, then by all means go ahead and send a patch to > Linus and see what he says about it ;)
I mean this: wmb() assumes that the data to be ordered are not in WC memory. rmb() assumes that the data can be in WC memory (lfence is only useful on WC --- it doesn't have any effect on other memory types).
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