On Thu, Feb 02, 2023 at 09:44:52PM +0100, Morten Brørup wrote: > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > Sent: Thursday, 2 February 2023 20.00 > > > > On Thu, Feb 02, 2023 at 09:43:58AM +0100, Morten Brørup wrote: > > > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com] > > > > Sent: Wednesday, 1 February 2023 22.41 > > > > > > > > On Wed, Feb 01, 2023 at 01:07:59AM +0000, Honnappa Nagarahalli > > wrote: > > > > > > > > > > > From: Thomas Monjalon <tho...@monjalon.net> > > > > > > Sent: Tuesday, January 31, 2023 4:42 PM > > > > > > > > > > > > Honnappa, please could you give your view on the future of > > atomics > > > > in DPDK? > > > > > Thanks Thomas, apologies it has taken me a while to get to this > > > > discussion. > > > > > > > > > > IMO, we do not need DPDK's own abstractions. APIs from > > stdatomic.h > > > > (stdatomics as is called here) already serve the purpose. These > > APIs > > > > are well understood and documented. > > > > > > > > i agree that whatever atomics APIs we advocate for should align > > with > > > > the > > > > standard C atomics for the reasons you state including implied > > > > semantics. > > > > > > > > > > > > > > For environments where stdatomics are not supported, we could > > have a > > > > stdatomic.h in DPDK implementing the same APIs (we have to support > > only > > > > _explicit APIs). This allows the code to use stdatomics APIs and > > when > > > > we move to minimum supported standard C11, we just need to get rid > > of > > > > the file in DPDK repo. > > > > > > Perhaps we can use something already existing, such as this: > > > https://android.googlesource.com/platform/bionic/+/lollipop- > > release/libc/include/stdatomic.h > > > > > > > > > > > my concern with this is that if we provide a stdatomic.h or > > introduce > > > > names > > > > from stdatomic.h it's a violation of the C standard. > > > > > > > > references: > > > > * ISO/IEC 9899:2011 sections 7.1.2, 7.1.3. > > > > * GNU libc manual > > > > https://www.gnu.org/software/libc/manual/html_node/Reserved- > > > > Names.html > > > > > > > > in effect the header, the names and in some instances namespaces > > > > introduced > > > > are reserved by the implementation. there are several reasons in > > the > > > > GNU libc > > > > manual that explain the justification for these reservations and if > > > > if we think about ODR and ABI compatibility we can conceive of > > others. > > > > > > I we are going to move to C11 soon, I consider the shim interim, and > > am inclined to ignore these warning factors. > > > > > > If we are not moving to C11 soon, I would consider these > > disadvantages more seriously. > > > > I think it's reasonable to assume that we are talking years here. > > > > We've had a few discussions about minimum C standard. I think my first > > mailing list exchanges about C99 was almost 2 years ago. Given that we > > still aren't on C99 now (though i know Bruce has a series up) indicates > > that progression to C11 isn't going to happen any time soon and even if > > it was the baseline we still can't just use it (reasons described > > later). > > > > Also, i'll point out that we seem to have accepted moving to C99 with > > one of the holdback compilers technically being non-conformant but it > > isn't blocking us because it provides the subset of C99 features > > without > > being conforming that we happen to be using. > > > > > > > > > > > > > i'll also remark that the inter-mingling of names from the POSIX > > > > standard implicitly exposed as a part of the EAL public API has > > been > > > > problematic for portability. > > > > > > This is a very important remark, which should be considered > > carefully! Tyler has firsthand experience with DPDK portability. If he > > thinks porting to Windows is going to be a headache if we expose the > > stdatomic.h API, we must listen! So, what is your gut feeling here, > > Tyler? > > > > I think this is even more of a concern with language standard than it > > is > > with a platform standard. Because the language standard is used across > > platforms. > > > > On the surface it looks appealing to just go through all the dpdk code > > one last time and #include <stdatomic.h> and directly depend on names > > that "look" standard. In practice though we aren't depending on the > > toolchain / libc surface we are binding ourselves to the shim and the > > implementation it provides. > > > > This is aside from the mechanics of making it work in the different > > contexts we now have to care about. Here is a story of how things > > become tricky. > > > > When i #include <stdatomic.h> which one gets used if the implementation > > provides one? Let's force our stdatomic.h > > > > Now i need to force the build system to prefer my shim header? Keeping > > in mind that the presence of a libc stdatomic.h does not mean that the > > toolchain in fact supports standard atomics. Okay, that's under our > > control by editing some meson.build files maybe it isn't so bad but... > > > > It seems my application also has to do the same in their build system > > now because... > > > > The type definitions (size, alignment) and code generated from the > > body of inline functions as seen by the application built translation > > units may differ from those in the dpdk translation units if they don't > > use our header. The potential for ABI compat problems is increasing but > > maybe it is managable? it can be worse... > > > > We can't limit our scope to thinking that there is just an > > application (a single binary) and dpdk. Complex applications will > > invariably depend on other libraries and if the application needs to > > interface with those compatibily at the ABI level using standard > > atomics > > then we've made it very difficult since the application has to choose > > to > > use our conflicting named atomic types which may not be compatible or > > the real standard atomics. They can of course produce horrible shims > > of their own to interoperate. > > > > We need consistency across the entire binary at runtime and i don't > > think it's practical to say that anyone who uses dpdk has to compile > > their whole world with our shim. So dealing with all this complexity > > for the sake of asthetics "looking" like the standard api seems kind > > of not worth it. Sure it saves having to deprecate later and one last > > session of shotgun surgery but that's kind of all we get. > > > > Don't think i'm being biased in favor of windows/msvc here. From the > > perspective of the windows/msvc combination i intend to use only the > > standard C ABI provided by the implementation. I have no intention of > > trying to introduce support for the current ABI that doesn't use the > > standard atomic types. my discouraging of this approach is about > > avoiding > > subtle to detect but very painful problems on > > {linux,unix}/compiler<version> > > combinations that already have a shipped/stable ABI. > > > > > > > > > > let's discuss this from here. if there's still overwhelming desire > > to > > > > go > > > > this route then we'll just do our best. > > > > > > > > ty > > > > > > I have a preference for exposing the stdatomic.h API. Tyler listed > > the disadvantages above. (I also have a preference for moving to C11 > > soon.) > > > > I am eager to see this happen, but as explained in my original proposal > > it doesn't eliminate the need for an abstraction. Unless we are willing > > to break our compatibility promises and potentially take a performance > > hit on some platform/compiler combinations which as i understand is not > > acceptable. > > > > > > > > Exposing a 1:1 similar API with RTE prefixes would also be acceptable > > for me. The disadvantage is that the names are different than the C11 > > names, which might lead to some confusion. And from an ABI stability > > perspective, such an important API should not be marked experimental. > > This means that years will pass before we can get rid of it again, due > > to ABI stability policies. > > > > I think the key to success with rte_ prefixed names is making > > absolutely > > sure we mirror the semantics and types in the standard. > > > > I will point out one bit of fine print here is that we will not support > > atomic operations on struct/union types (something the standard > > supports). > > With the rte_ namespace i think this becomes less ambiguous, if we > > present > > standard C names though what's to avoid the confusion? Aside from it > > fails > > to compile with one compiler vs another. > > > > I agree that this may be around for years. But how many years depends a > > lot on how long we have to maintain compatibility for the existing > > platform/compiler combinations that can't (and aren't enabled) to use > > the standard. > > > > Even if we introduced standard names we still have to undergo some kind > > of mutant deprecation process to get the world to recompile everything > > against the actual standard, so it doesn't give us forward > > compatibility. > > > > Let me know what folks would like to do, i guess i'm firmly leaned > > toward no-shim and just rte_ explicit. But as a community i'll pursue > > whatever you decide. > > > > Thanks! > > Tyler is making a very strong case here. > > I have changed my mind, and now support Tyler's approach. >
Having read through the whole thread a second time, I am starting to realise how complex a problem this could be. From what I read, I would tend towards the opinion that we shouldn't provide any atomics in DPDK at all, and just rely on the C standard ones. The main complication in any solution I suspect is going to be the use of atomics in static inline functions we have in our header files. /Bruce
