Messages in this thread | | | Date | Sat, 5 Jun 2021 10:57:39 -0400 | From | Alan Stern <> | Subject | Re: [RFC] LKMM: Add volatile_if() |
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On Fri, Jun 04, 2021 at 03:19:11PM -0700, Linus Torvalds wrote: > Now, part of this is that I do think that in *general* we should never > use this very suble load-cond-store pattern to begin with. We should > strive to use more smp_load_acquire() and smp_store_release() if we > care about ordering of accesses. They are typically cheap enough, and > if there's much of an ordering issue, they are the right things to do. > > I think the whole "load-to-store ordering" subtle non-ordered case is > for very very special cases, when you literally don't have a general > memory ordering, you just have an ordering for *one* very particular > access. Like some of the very magical code in the rw-semaphore case, > or that smp_cond_load_acquire(). > > IOW, I would expect that we have a handful of uses of this thing. And > none of them have that "the conditional store is the same on both > sides" pattern, afaik. > > And immediately when the conditional store is different, you end up > having a dependency on it that orders it. > > But I guess I can accept the above made-up example as an "argument", > even though I feel it is entirely irrelevant to the actual issues and > uses we have.
Indeed, the expansion of the currently proposed version of
volatile_if (A) { B; } else { C; }
is basically the same as
if (A) { barrier(); B; } else { barrier(); C; }
which is just about as easy to write by hand. (For some reason my fingers don't like typing "volatile_"; the letters tend to get scrambled.)
So given that:
1. Reliance on control dependencies is uncommon in the kernel, and
2. The loads in A could just be replaced with load_acquires at a low penalty (or store-releases could go into B and C),
it seems that we may not need volatile_if at all! The only real reason for having it in the first place was to avoid the penalty of load-acquire on architectures where it has a significant cost, when the control dependency would provide the necessary ordering for free. Such architectures are getting less and less common.
Alan
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