[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 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: > > On Thu, Jan 14, 2016 at 02:55:10PM -0800, Paul E. McKenney wrote: > > > On Thu, Jan 14, 2016 at 01:36:50PM -0800, Leonid Yegoshin wrote: > > > > On 01/14/2016 01:29 PM, Paul E. McKenney wrote: > > > > > > > > > >>On 01/14/2016 12:34 PM, Paul E. McKenney wrote: > > > > >>> > > > > >>>The WRC+addr+addr is OK because data dependencies are not required > > > > >>>to be > > > > >>>transitive, in other words, they are not required to flow from one > > > > >>>CPU to > > > > >>>another without the help of an explicit memory barrier. > > > > >>I don't see any reliable way to fit WRC+addr+addr into "DATA > > > > >>DEPENDENCY BARRIERS" section recommendation to have data dependency > > > > >>barrier between read of a shared pointer/index and read the shared > > > > >>data based on that pointer. If you have this two reads, it doesn't > > > > >>matter the rest of scenario, you should put the dependency barrier > > > > >>in code anyway. If you don't do it in WRC+addr+addr scenario then > > > > >>after years it can be easily changed to different scenario which > > > > >>fits some of scenario in "DATA DEPENDENCY BARRIERS" section and > > > > >>fails. > > > > >The trick is that lockless_dereference() contains an > > > > >smp_read_barrier_depends(): > > > > > > > > > >#define lockless_dereference(p) \ > > > > >({ \ > > > > > typeof(p) _________p1 = READ_ONCE(p); \ > > > > > smp_read_barrier_depends(); /* Dependency order vs. p above. */ > > > > > \ > > > > > (_________p1); \ > > > > >}) > > > > > > > > > >Or am I missing your point? > > > > > > > > WRC+addr+addr has no any barrier. lockless_dereference() has a > > > > barrier. I don't see a common points between this and that in your > > > > answer, sorry. > > > > > > Me, I am wondering what WRC+addr+addr has to do with anything at all. > > > > 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? 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? ;-) Thanx, Paul PS. And yes, working through this does help me understand the benefits of fake dependencies. Why do you ask? ;-) ------------------------------------------------------------------------ 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) _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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