[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h
On Tue, Jan 26, 2016 at 12:10:10PM +0000, Will Deacon wrote: > On Mon, Jan 25, 2016 at 05:06:46PM -0800, Paul E. McKenney wrote: > > On Mon, Jan 25, 2016 at 02:41:34PM +0000, Will Deacon wrote: > > > On Fri, Jan 15, 2016 at 11:28:45AM -0800, Paul E. McKenney wrote: > > > > On Fri, Jan 15, 2016 at 09:54:01AM -0800, Paul E. McKenney wrote: > > > > > On Fri, Jan 15, 2016 at 10:24:32AM +0000, Will Deacon wrote: > > > > > > See my earlier reply [1] (but also, your WRC Linux example looks > > > > > > more > > > > > > like a variant on WWC and I couldn't really follow it). > > > > > > > > > > I will revisit my WRC Linux example. And yes, creating litmus tests > > > > > that use non-fake dependencies is still a bit of an undertaking. :-/ > > > > > I am sure that it will seem more natural with time and experience... > > > > > > > > Hmmm... You are quite right, I did do WWC. I need to change cpu2()'s > > > > last access from a store to a load to get WRC. Plus the levels of > > > > indirection definitely didn't match up, did they? > > > > > > Nope, it was pretty baffling! > > > > "It is a service that I provide." ;-) > > > > > > struct foo { > > > > struct foo *next; > > > > }; > > > > struct foo a; > > > > struct foo b; > > > > struct foo c = { &a }; > > > > struct foo d = { &b }; > > > > struct foo x = { &c }; > > > > struct foo y = { &d }; > > > > struct foo *r1, *r2, *r3; > > > > > > > > void cpu0(void) > > > > { > > > > WRITE_ONCE(x.next, &y); > > > > } > > > > > > > > void cpu1(void) > > > > { > > > > r1 = lockless_dereference(x.next); > > > > WRITE_ONCE(r1->next, &x); > > > > } > > > > > > > > void cpu2(void) > > > > { > > > > r2 = lockless_dereference(y.next); > > > > r3 = READ_ONCE(r2->next); > > > > } > > > > > > > > In this case, it is legal to end the run with: > > > > > > > > r1 == &y && r2 == &x && r3 == &c > > > > > > > > Please see below for a ppcmem litmus test. > > > > > > > > So, did I get it right this time? ;-) > > > > > > The code above looks correct to me (in that it matches WRC+addrs), > > > but your litmus test: > > > > > > > PPC WRCnf+addrs > > > > "" > > > > { > > > > 0:r2=x; 0:r3=y; > > > > 1:r2=x; 1:r3=y; > > > > 2:r2=x; 2:r3=y; > > > > c=a; d=b; x=c; y=d; > > > > } > > > > P0 | P1 | P2 ; > > > > stw r3,0(r2) | lwz r8,0(r2) | lwz r8,0(r3) ; > > > > | stw r2,0(r3) | lwz r9,0(r8) ; > > > > exists > > > > (1:r8=y /\ 2:r8=x /\ 2:r9=c) > > > > > > Seems to be missing the address dependency on P1. > > > > You are quite correct! How about the following? > > I think that's it! > > > As before, both herd and ppcmem say that the cycle is allowed, as > > expected, given non-transitive ordering. To prohibit the cycle, P1 > > needs a suitable memory-barrier instruction. > > > > ------------------------------------------------------------------------ > > > > PPC WRCnf+addrs > > "" > > { > > 0:r2=x; 0:r3=y; > > 1:r2=x; 1:r3=y; > > 2:r2=x; 2:r3=y; > > c=a; d=b; x=c; y=d; > > } > > P0 | P1 | P2 ; > > stw r3,0(r2) | lwz r8,0(r2) | lwz r8,0(r3) ; > > | stw r2,0(r8) | lwz r9,0(r8) ; > > exists > > (1:r8=y /\ 2:r8=x /\ 2:r9=c) > > Agreed. OK, thank you! Would you agree that it would be good to replace the current xor-based fake-dependency litmus tests with tests having real dependencies? Thanx, Paul _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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