Re: [PATCH] eal: introduce atomics abstraction

[Tyler Retzlaff <roretzla@linux.microsoft.com>]

On Fri, Feb 10, 2023 at 05:30:00AM +0000, Honnappa Nagarahalli wrote:
> <snip>
> 
> > On Thu, Feb 09, 2023 at 12:16:38AM +0000, Honnappa Nagarahalli wrote:
> > > <snip>
> > >
> > > > > > > >
> > > > > > > > >
> > > > > > > > > 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.
> > > > > > > >
> > > > > > > > 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
> > > > > > > Wouldn't this apply only after the particular APIs were introduced?
> > > > > > i.e. it should not apply if the compiler does not support stdatomics.
> > > > > >
> > > > > > yeah, i agree they're being a bit wishy washy in the wording,
> > > > > > but i'm not convinced glibc folks are documenting this as
> > > > > > permissive guidance against.
> > > > > >
> > > > > > >
> > > > > > > > manual that explain the justification for these reservations
> > > > > > > > and if
> > > > > > if we think
> > > > > > > > about ODR and ABI compatibility we can conceive of others.
> > > > > > > >
> > > > > > > > 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.
> > > > > > > These should be exposed as EAL APIs only when compiled with a
> > > > > > compiler that does not support stdatomics.
> > > > > >
> > > > > > you don't necessarily compile dpdk, the application or its other
> > > > > > dynamically linked dependencies with the same compiler at the
> > > > > > same time.
> > > > > > i.e. basically the model of any dpdk-dev package on any linux
> > > > > > distribution.
> > > > > >
> > > > > > if dpdk is built without real stdatomic types but the
> > > > > > application has to interoperate with a different kit or library
> > > > > > that does they would be forced to dance around dpdk with their
> > > > > > own version of a shim to hide our faked up stdatomics.
> > > > > >
> > > > >
> > > > > So basically, if we want a binary DPDK distribution to be
> > > > > compatible with a
> > > > separate application build environment, they both have to implement
> > > > atomics the same way, i.e. agree on the ABI for atomics.
> > > > >
> > > > > Summing up, this leaves us with only two realistic options:
> > > > >
> > > > > 1. Go all in on C11 stdatomics, also requiring the application
> > > > > build
> > > > environment to support C11 stdatomics.
> > > > > 2. Provide our own DPDK atomics library.
> > > > >
> > > > > (As mentioned by Tyler, the third option - using C11 stdatomics
> > > > > inside DPDK, and requiring a build environment without C11
> > > > > stdatomics to implement a shim - is not realistic!)
> > > > >
> > > > > I strongly want atomics to be available for use across inline and
> > > > > compiled
> > > > code; i.e. it must be possible for both compiled DPDK functions and
> > > > inline functions to perform atomic transactions on the same atomic variable.
> > > >
> > > > i consider it a mandatory requirement. i don't see practically how
> > > > we could withdraw existing use and even if we had clean way i don't
> > > > see why we would want to. so this item is defintely settled if you were
> > concerned.
> > > I think I agree here.
> > >
> > > >
> > > > >
> > > > > So either we upgrade the DPDK build requirements to support C11
> > > > > (including
> > > > the optional stdatomics), or we provide our own DPDK atomics.
> > > >
> > > > i think the issue of requiring a toolchain conformant to a specific
> > > > standard is a separate matter because any adoption of C11 standard
> > > > atomics is a potential abi break from the current use of intrinsics.
> > > I am not sure why you are calling it as ABI break. Referring to [1], I just see
> > wrappers around intrinsics (though [2] does not use the intrinsics).
> > >
> > > [1]
> > > https://github.com/gcc-mirror/gcc/blob/master/gcc/ginclude/stdatomic.h
> > > [2]
> > > https://github.com/llvm-mirror/clang/blob/master/lib/Headers/stdatomic
> > > .h
> > 
> > it's a potential abi break because atomic types are not the same types as their
> > corresponding integer types etc.. (or at least are not guaranteed to be by all
> > implementations of c as an abstract language).
> > 
> >     ISO/IEC 9899:2011
> > 
> >     6.2.5 (27)
> >     Further, there is the _Atomic qualifier. The presence of the _Atomic
> >     qualifier designates an atomic type. The size, representation, and alignment
> >     of an atomic type need not be the same as those of the corresponding
> >     unqualified type.
> > 
> >     7.17.6 (3)
> >     NOTE The representation of atomic integer types need not have the same size
> >     as their corresponding regular types. They should have the same size
> > whenever
> >     possible, as it eases effort required to port existing code.
> > 
> > i use the term `potential abi break' with intent because for me to assert in
> > absolute terms i would have to evaluate the implementation of every current
> > and potential future compilers atomic vs non-atomic types. this as i'm sure you
> > understand is not practical, it would also defeat the purpose of moving to a
> > standard. therefore i rely on the specification prescribed by the standard not
> > the detail of a specific implementation.
> Can we say that the platforms 'supported' by DPDK today do not have this problem? Any future platforms that will come to DPDK have to evaluate this.

sadly i don't think we can. i believe in an earlier post i linked a bug
filed on gcc that shows that clang / gcc were producing different
layout than the equivalent non-atomic type.

> 
> > 
> > 
> > > > the abstraction (whatever namespace it resides) allows the existing
> > > > toolchain/platform combinations to maintain compatibility by
> > > > defaulting to current non-standard intrinsics.
> > > How about using the intrinsics (__atomic_xxx) name space for abstraction?
> > This covers the GCC and Clang compilers.

i haven't investigated fully but there are usages of these intrinsics
that indicate there may be undesirable difference between clang and gcc
versions. the hint is there seems to be conditionally compiled code
under __clang__ when using some __atomic's.

for the purpose of this discussion clang just tries to look like gcc so
i don't regard them as being different compilers for the purpose of this
discussion.

> > 
> > the namespace starting with `__` is also reserved for the implementation.
> > this is why compilers gcc/clang/msvc place name their intrinsic and builtin
> > functions starting with __ to explicitly avoid collision with the application
> > namespace.

> Agreed. But, here we are considering '__atomic_' specifically (i.e. not just '__')

i don't understand the confusion __atomic is within the __ namespace
that is reserved.

let me ask this another way, what benefit do you see to trying to
overlap with the standard namespace? the only benefit i can see is that
at some point in the future it avoids having to perform a mechanical
change to eventually retire the abstraction once all platform/toolchains
support standard atomics. i.e. basically s/rte_atomic/atomic/g

is there another benefit i'm missing?

> 
> > 
> >     ISO/IEC 9899:2011
> > 
> >     7.1.3 (1)
> >     All identifiers that begin with an underscore and either an uppercase
> >     letter or another underscore are always reserved for any use.
> > 
> >     ...
> > 
> > > If there is another platform that uses the same name space for something
> > else, I think DPDK should not be supporting that platform.
> > 
> > that's effectively a statement excluding windows platform and all non-gcc
> > compilers from ever supporting dpdk.
> Apologies, I did not understand your comment on windows platform. Do you mean to say a compiler for windows platform uses '__atomic_xxx' name space to provide some other functionality (and hence it would get excluded)? 

i mean dpdk can never fully be supported without msvc except for
statically linked builds which are niche and limit it too severely for
many consumers to practically use dpdk. there are also many application
developers who would like to integrate dpdk but can't and telling them
their only choice is to re-port their entire application to clang isn't
feasible.

i can see no technical reason why we should be excluding a major
compiler in broad use if it is capable of building dpdk. msvc arguably
has some of the most sophisticated security features in the industry
and the use of those features is mandated by many of the customers who
might deploy dpdk applications on windows.

> Clang supports these intrinsics. I am not sure about the merit of supporting other non-gcc compilers. May be a topic Techboard discussion.
> 
> > 
> > > What problems do you see?
> > 
> > i'm fairly certain at least one other compiler uses the __atomic namespace but
> Do you mean __atomic namespace is used for some other purpose?
> 
> > it would take me time to check, the most notable potential issue that comes to
> > mind is if such an intrinsic with the same name is provided in a different
> > implementation and has either regressive code generation or different
> > semantics it would be bad because it is intrinsic you can't just hack around it
> > with #undef __atomic to shim in a semantically correct version.
> I do not think we should worry about regressive code generation problem. It should be fixed by that compiler.
> Different semantics is something we need to worry about. It would be good to find out more about a compiler that does this.

again, this is about portability it's about potential not that we can
find an example.

> 
> > 
> > how about this, is there another possible namespace you might suggest that
> > conforms or doesn't conflict with the the rules defined in ISO/IEC 9899:2011
> > 7.1.3 i think if there were that would satisfy all of my concerns related to
> > namespaces.
> > 
> > keep in mind the point of moving to a standard is to achieve portability so if we
> > do things that will regress us back to being dependent on an implementation
> > we haven't succeeded. that's all i'm trying to guarantee here.
> Agree. We are trying to solve a problem that is temporary. I am trying to keep the problem scope narrow which might help us push to adopt the standard sooner.

i do wish we could just target the standard but unless we are willing to
draw a line and say no more non std=c11 and also we potentially break
the abi we are talking years. i don't think it is reasonable to block
progress for years, so i'm offering a transitional path. it's an
evolution over time that we have to manage.

> 
> > 
> > i feel like we are really close on this discussion, if we can just iron this issue out
> > we can probably get going on the actual changes.
> > 
> > thanks for the consideration.
> > 
> > >
> > > >
> > > > once in place it provides an opportunity to introduce new
> > > > toolchain/platform combinations and enables an opt-in capability to
> > > > use stdatomics on existing toolchain/platform combinations subject
> > > > to community discussion on how/if/when.
> > > >
> > > > it would be good to get more participants into the discussion so
> > > > i'll cc techboard for some attention. i feel like the only area that
> > > > isn't decided is to do or not do this in rte_ namespace.
> > > >
> > > > i'm strongly in favor of rte_ namespace after discussion, mainly due
> > > > to to disadvantages of trying to overlap with the standard namespace
> > > > while not providing a compatible api/abi and because it provides
> > > > clear disambiguation of that difference in semantics and compatibility with
> > the standard api.
> > > >
> > > > so far i've noted the following
> > > >
> > > > * we will not provide the non-explicit apis.
> > > +1
> > >
> > > > * we will make no attempt to support operate on struct/union atomics
> > > >   with our apis.
> > > +1
> > >
> > > > * we will mirror the standard api potentially in the rte_ namespace to
> > > >   - reference the standard api documentation.
> > > >   - assume compatible semantics (sans exceptions from first 2 points).
> > > >
> > > > my vote is to remove 'potentially' from the last point above for
> > > > reasons previously discussed in postings to the mail thread.
> > > >
> > > > thanks all for the discussion, i'll send up a patch removing
> > > > non-explicit apis for viewing.
> > > >
> > > > ty