> -----Original Message----- > From: Boqun Feng [mailto:boqun.f...@gmail.com] > Sent: Wednesday, November 15, 2017 5:19 PM > To: Daniel Lustig <dlus...@nvidia.com> > Cc: Palmer Dabbelt <pal...@dabbelt.com>; will.dea...@arm.com; Arnd > Bergmann <a...@arndb.de>; Olof Johansson <o...@lixom.net>; linux- > ker...@vger.kernel.org; patc...@groups.riscv.org; pet...@infradead.org > Subject: Re: [patches] Re: [PATCH v9 05/12] RISC-V: Atomic and Locking Code > > On Wed, Nov 15, 2017 at 11:59:44PM +0000, Daniel Lustig wrote: > > > On Wed, 15 Nov 2017 10:06:01 PST (-0800), will.dea...@arm.com wrote: > > >> On Tue, Nov 14, 2017 at 12:30:59PM -0800, Palmer Dabbelt wrote: > > >> > On Tue, 24 Oct 2017 07:10:33 PDT (-0700), will.dea...@arm.com > wrote: > > >> >>On Tue, Sep 26, 2017 at 06:56:31PM -0700, Palmer Dabbelt wrote: > > > > > > > > Hi Palmer, > > > > > > > >> >>+ATOMIC_OPS(add, add, +, i, , _relaxed) > > > >> >>+ATOMIC_OPS(add, add, +, i, .aq , _acquire) ATOMIC_OPS(add, > > > >> >>+add, > > > >> >>++, i, .rl , _release) > > > >> >>+ATOMIC_OPS(add, add, +, i, .aqrl, ) > > > >> > > > > >> >Have you checked that .aqrl is equivalent to "ordered", since > > > >> >there are interpretations where that isn't the case. Specifically: > > > >> > > > > >> >// all variables zero at start of time > > > >> >P0: > > > >> >WRITE_ONCE(x) = 1; > > > >> >atomic_add_return(y, 1); > > > >> >WRITE_ONCE(z) = 1; > > > >> > > > > >> >P1: > > > >> >READ_ONCE(z) // reads 1 > > > >> >smp_rmb(); > > > >> >READ_ONCE(x) // must not read 0 > > > >> > > > >> I haven't. We don't quite have a formal memory model specification > yet. > > > >> I've added Daniel Lustig, who is creating that model. He should > > > >> have a better idea > > > > > > > > Thanks. You really do need to ensure that, as it's heavily relied upon. > > > > > > I know it's the case for our current processors, and I'm pretty sure > > > it's the case for what's formally specified, but we'll have to wait > > > for the spec in order to prove it. > > > > I think Will is right. In the current spec, using .aqrl converts an > > RCpc load or store into an RCsc load or store, but the acquire(-RCsc) > > annotation still only applies to the load part of the atomic, and the > > release(-RCsc) annotation applies only to the store part of the atomic. > > > > Why is that? Picture an machine which implements AMOs using something > > that looks more like an LR/SC under the covers, or one that uses cache > > line locking, or anything else along those same lines. In some such > > machines, there could be a window between lock/reserve and > > unlock/store-conditional where other later stores could squeeze into, and > that would break Will's example among others. > > > > It's likely the same reasoning that causes ARM to use a trailing dmb > > here, rather than just using ldaxr/stlxr. Is that right Will? I know > > that's LL/SC and this particular cases uses AMOADD, but it's the same > > principle. Well, at least according to how we have it in the current memory > model draft. > > > > Also, RISC-V currently prefers leading fence mappings, so I think the > > result here, for atomic_add_return() for example, should be this: > > > > fence rw,rw > > amoadd.aq ... > > > > Hmm.. if atomic_add_return() is implemented like that, how about the > following case: > > {x=0, y=0} > > P1: > > r1 = atomic_add_return(&x, 1); // r1 == 0, x will 1 afterwards > WRITE_ONCE(y, 1); > > P2: > > r2 = READ_ONCE(y); // r2 = 1 > smp_rmb(); > r3 = atomic_read(&x); // r3 = 0? > > , could this result in r1 == 1 && r2 == 1 && r3 == 0? Given you said .aq only > effects the load part of AMO, and I don't see anything here preventing the > reordering between store of y and the store part of the AMO on P1. > > Note: we don't allow (r1 == 1 && r2 == 1 && r3 == 0) in above case for linux > kernel. Please see Documentation/atomic_t.txt: > > "Fully ordered primitives are ordered against everything prior and everything > subsequent. Therefore a fully ordered primitive is like having an smp_mb() > before and an smp_mb() after the primitive."
Yes, you're right Boqun. Good catch, and sorry for over-optimizing too quickly. In that case, maybe we should just start out having a fence on both sides for now, and then we'll discuss offline whether we want to change the model's behavior here. Dan
