[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 09:55:54AM +0100, Peter Zijlstra wrote: > On Thu, Jan 14, 2016 at 01:29:13PM -0800, Paul E. McKenney wrote: > > So smp_mb() provides transitivity, as do pairs of smp_store_release() > > and smp_read_acquire(), > > But they provide different grades of transitivity, which is where all > the confusion lays. > > smp_mb() is strongly/globally transitive, all CPUs will agree on the order. > > Whereas the RCpc release+acquire is weakly so, only the two cpus > involved in the handover will agree on the order. Good point! Using grace periods in place of smp_mb() also provides strong/global transitivity, but also insanely high latencies. ;-) The patch below updates Documentation/memory-barriers.txt to define local vs. global transitivity. The corresponding ppcmem litmus test is included below as well. Should we start putting litmus tests for the various examples somewhere, perhaps in a litmus-tests directory within each participating architecture? I have a pile of powerpc-related litmus tests on my laptop, but they probably aren't doing all that much good there. Thanx, Paul ------------------------------------------------------------------------ PPC local-transitive "" { 0:r1=1; 0:r2=u; 0:r3=v; 0:r4=x; 0:r5=y; 0:r6=z; 1:r1=1; 1:r2=u; 1:r3=v; 1:r4=x; 1:r5=y; 1:r6=z; 2:r1=1; 2:r2=u; 2:r3=v; 2:r4=x; 2:r5=y; 2:r6=z; 3:r1=1; 3:r2=u; 3:r3=v; 3:r4=x; 3:r5=y; 3:r6=z; } P0 | P1 | P2 | P3 ; lwz r9,0(r4) | lwz r9,0(r5) | lwz r9,0(r6) | stw r1,0(r3) ; lwsync | lwsync | lwsync | sync ; stw r1,0(r2) | lwz r8,0(r3) | stw r1,0(r7) | lwz r9,0(r2) ; lwsync | lwz r7,0(r2) | | ; stw r1,0(r5) | lwsync | | ; | stw r1,0(r6) | | ; exists (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r8=0 /\ 3:r9=0) *) (* (0:r9=1 /\ 1:r9=1 /\ 2:r9=1) *) (* (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0) *) (0:r9=0 /\ 1:r9=1 /\ 2:r9=1 /\ 1:r7=0) ------------------------------------------------------------------------ commit 2cb4e83a1b5c89c8e39b8a64bd89269d05913e41 Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> Date: Fri Jan 15 09:30:42 2016 -0800 documentation: Distinguish between local and global transitivity The introduction of smp_load_acquire() and smp_store_release() had the side effect of introducing a weaker notion of transitivity: The transitivity of full smp_mb() barriers is global, but that of smp_store_release()/smp_load_acquire() chains is local. This commit therefore introduces the notion of local transitivity and gives an example. Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Reported-by: Will Deacon <will.deacon@xxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index c66ba46d8079..d8109ed99342 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -1318,8 +1318,82 @@ or a level of cache, CPU 2 might have early access to CPU 1's writes. General barriers are therefore required to ensure that all CPUs agree on the combined order of CPU 1's and CPU 2's accesses. -To reiterate, if your code requires transitivity, use general barriers -throughout. +General barriers provide "global transitivity", so that all CPUs will +agree on the order of operations. In contrast, a chain of release-acquire +pairs provides only "local transitivity", so that only those CPUs on +the chain are guaranteed to agree on the combined order of the accesses. +For example, switching to C code in deference to Herman Hollerith: + + int u, v, x, y, z; + + void cpu0(void) + { + r0 = smp_load_acquire(&x); + WRITE_ONCE(u, 1); + smp_store_release(&y, 1); + } + + void cpu1(void) + { + r1 = smp_load_acquire(&y); + r4 = READ_ONCE(v); + r5 = READ_ONCE(u); + smp_store_release(&z, 1); + } + + void cpu2(void) + { + r2 = smp_load_acquire(&z); + smp_store_release(&x, 1); + } + + void cpu3(void) + { + WRITE_ONCE(v, 1); + smp_mb(); + r3 = READ_ONCE(u); + } + +Because cpu0(), cpu1(), and cpu2() participate in a local transitive +chain of smp_store_release()/smp_load_acquire() pairs, the following +outcome is prohibited: + + r0 == 1 && r1 == 1 && r2 == 1 + +Furthermore, because of the release-acquire relationship between cpu0() +and cpu1(), cpu1() must see cpu0()'s writes, so that the following +outcome is prohibited: + + r1 == 1 && r5 == 0 + +However, the transitivity of release-acquire is local to the participating +CPUs and does not apply to cpu3(). Therefore, the following outcome +is possible: + + r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 + +Although cpu0(), cpu1(), and cpu2() will see their respective reads and +writes in order, CPUs not involved in the release-acquire chain might +well disagree on the order. This disagreement stems from the fact that +the weak memory-barrier instructions used to implement smp_load_acquire() +and smp_store_release() are not required to order prior stores against +subsequent loads in all cases. This means that cpu3() can see cpu0()'s +store to u as happening -after- cpu1()'s load from v, even though +both cpu0() and cpu1() agree that these two operations occurred in the +intended order. + +However, please keep in mind that smp_load_acquire() is not magic. +In particular, it simply reads from its argument with ordering. It does +-not- ensure that any particular value will be read. Therefore, the +following outcome is possible: + + r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0 + +Note that this outcome can happen even on a mythical sequentially +consistent system where nothing is ever reordered. + +To reiterate, if your code requires global transitivity, use general +barriers throughout. ======================== _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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