On Fri, Aug 31, 2018 at 10:52:54AM -0400, Alan Stern wrote: > On Fri, 31 Aug 2018, Andrea Parri wrote: > > > On Thu, Aug 30, 2018 at 05:31:32PM -0400, Alan Stern wrote: > > > On Thu, 30 Aug 2018, Andrea Parri wrote: > > > > > > > > All the architectures supported by the Linux kernel (including RISC-V) > > > > > do provide this ordering for locks, albeit for varying reasons. > > > > > Therefore this patch changes the model in accordance with the > > > > > developers' wishes. > > > > > > > > > > Signed-off-by: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> > > > > > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> > > > > > Reviewed-by: Will Deacon <will.deacon@xxxxxxx> > > > > > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx> > > > > > > > > Round 2 ;-), I guess... Let me start from the uncontroversial points: > > > > > > > > 1) being able to use the LKMM to reason about generic locking code > > > > is useful and desirable (paraphrasing Peter in [1]); > > > > > > > > 2) strengthening the ordering requirements of such code isn't going > > > > to boost performance (that's "real maths"). > > > > > > > > This patch is taking (1) away from us and it is formalizing (2), with > > > > almost _no_ reason (no reason at all, if we stick to the commit msg.). > > > > > > That's not quite fair. Generic code isn't always universally > > > applicable; some of it is opt-in -- meant only for the architectures > > > that can support it. In general, the LKMM allows us to reason about > > > higher abstractions (such as locking) at a higher level, without > > > necessarily being able to verify the architecture-specific details of > > > the implementations. > > > > No, generic code is "universally applicable" by definition; see below > > for more on this point. > > Then perhaps we should be talking about "partially generic" code; i.e., > code that applies to many but not all architectures. Perhaps. I call this "non-generic". ;-) > > > > > In [2], Will wrote: > > > > > > > > "[...] having them [the RMWs] closer to RCsc[/to the semantics of > > > > locks] would make it easier to implement and reason about generic > > > > locking implementations (i.e. reduce the number of special ordering > > > > cases and/or magic barrier macros)" > > > > > > > > "magic barrier macros" as in "mmh, if we accept this patch, we _should_ > > > > be auditing the various implementations/code to decide where to place a > > > > > > > > smp_barrier_promote_ordinary_release_acquire_to_unlock_lock()" ;-) > > > > > > > > or the like, and "special ordering cases" as in "arrgh, (otherwise) we > > > > are forced to reason on a per-arch basis while looking at generic code". > > > > > > Currently the LKMM does not permit architecture-specific reasoning. It > > > would have to be extended (in a different way for each architecture) > > > first. > > > > Completely agreed; that's why I said that this patch is detrimental to > > the applicability of the LKMM... > > This ignores the attitude of the kernel developers who want locking to > have the stronger RCtso semantics. From their point of view, the patch > enhances the LKMM's applicability. Please stop. We spent weeks discussing how to make such a transition properly, and I suggested ways to do this above and in other threads. (Again, your commit message says nothing about that "enhancement"...) > > > > For example, one could use herd's POWER model combined with the POWER > > > compilation scheme and the POWER-specific implementation of spinlocks > > > for such reasoning. The LKMM alone is not sufficient. > > > > > > Sure, programming and reasoning about the kernel would be easier if all > > > architectures were the same. Unfortunately, we (and the kernel) have > > > to live in the real world. > > > > > > > (Remark: ordinary release/acquire are building blocks for code such as > > > > qspinlock, (q)rwlock, mutex, rwsem, ... and what else??). > > > > > > But are these building blocks used the same way for all architectures? > > > > The more, the better! (because then we have the LKMM tools) > > > > We already discussed the "fast path" example: the fast paths of the > > above all resemble: > > > > *_lock(s): atomic_cmpxchg_acquire(&s->val, UNLOCKED_VAL, LOCKED_VAL) ... > > *_unlock(s): ... atomic_set_release(&s->val, UNLOCKED_VAL) > > > > When I read this code, I think "Of course." (unless some arch. has > > messed the implementation of cmpxchg_* up, which can happen...); but > > then I read the subject line of this patch and I think "Wait, what?". > > > > You can argue that this is not generic code, sure; but why on Earth > > would you like to do so?! > > Because the code might not work! On RISC-V, for example, the > implementation of ordinary release/acquire is currently not as strong > as atomic release/acquire. No doubt it "might not work". ;-) Concerning the RISC-V implementation, I almost (re)did that, so at least you know who to blame if what you are saying turned out to be correct. > > Yes, it's true that implementing locks with atomic_cmpxchg_acquire > should be correct on all existing architectures. And Paul has invited > a patch to modify the LKMM accordingly. If you feel that such a change > would be a useful enhancement to the LKMM's applicability, please write > it. Hey, it's your patch that is introducing concerns! "my patch" would partially _but_ substantially revert yours (see earlier discussions): I'm still hoping that we'll be able to find an agreement... Andrea > > Alan >
