On Wed, Aug 16, 2023 at 10:13:22PM +0200, Morten Brørup wrote:
> > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com]
> > Sent: Monday, 14 August 2023 19.47
> >
> > On Mon, Aug 14, 2023 at 10:00:49AM +0200, Morten Brørup wrote:
> > > > From: Tyler Retzlaff [mailto:roret...@linux.microsoft.com]
> > > > Sent: Friday, 11 August 2023 19.32
> > > >
> > > > Adapt the EAL public headers to use rte optional atomics API instead
> > of
> > > > directly using and exposing toolchain specific atomic builtin
> > intrinsics.
> > > >
> > > > Signed-off-by: Tyler Retzlaff <roret...@linux.microsoft.com>
> > > > ---
> > >
> > > [...]
> > >
> >
> > will fix the comments identified.
> >
> > >
> > > [...]
> > >
> > > > --- a/lib/eal/include/generic/rte_spinlock.h
> > > > +++ b/lib/eal/include/generic/rte_spinlock.h
> > > > @@ -29,7 +29,7 @@
> > > > * The rte_spinlock_t type.
> > > > */
> > > > typedef struct __rte_lockable {
> > > > - volatile int locked; /**< lock status 0 = unlocked, 1 = locked
> > > > */
> > > > + volatile int __rte_atomic locked; /**< lock status 0 =
> > > > unlocked, 1
> > =
> > > > locked */
> > >
> > > I think __rte_atomic should be before the type:
> > > volatile __rte_atomic int locked; /**< lock status [...]
> > > Alternatively (just mentioning it, I know we don't use this form):
> > > volatile __rte_atomic(int) locked; /**< lock status [...]
> > >
> > > Thinking of where you would put "const" might help.
>
> Regarding "const", I use the mental trick of reading from right-to-left when
> pointers are involved, e.g.:
>
> const int * * const x;
> ----5---- 4 3 --2-- 1
yes, i'm very familiar with where it can appear in the syntax and
applied. but it's always good to have someone summarize it like this for
the discussion.
>
> x(1) is a const(2) pointer(3) to a pointer(4) to a const int(5).
>
> And yes, treating "const int" as one word is cheating... formally it should
> be "int" "const", i.e. the reverse order; but that is not the convention, so
> I have learned to accept it.
it more often is the convention in c++, but i agree in c conventionally
people put the const first.
>
> > >
> > > Maybe your order is also correct, so it is a matter of preference.
> >
> > so for me what you suggest is the canonical convention for c and i did
> > initially try to make the change with this convention but ran into
> > trouble when using the keyword in a context used as a type specifier
> > and the type was incomplete.
> >
> > the rte_mcslock is a good example for illustration.
> >
> > // original struct
> > typedef struct rte_mcslock {
> > struct rte_mcslock *next;
> > ...
> > };
> >
> > it simply doesn't work / won't compile (at least with clang) which is
> > what drove me to use the less-often used syntax.
> >
> > typedef struct rte_mcslock {
> > _Atomic struct rte_mcslock *next;
> > ...
> > };
> >
> > In file included from ../app/test/test_mcslock.c:19:
> > ..\lib\eal\include\rte_mcslock.h:36:2: error: _Atomic cannot be
> > applied
> > to incomplete type 'struct rte_mcslock'
> > _Atomic struct rte_mcslock *next;
> > ^
> > ..\lib\eal\include\rte_mcslock.h:35:16: note: definition of 'struct
> > rte_mcslock' is not complete until the closing '}'
> > typedef struct rte_mcslock {
> > ^
> > 1 error generated.
> >
> > so i ended up choosing to use a single syntax by convention consistently
> > rather than using one for the exceptional case and one everywhere else.
> >
> > i think (based on our other thread of discussion) i would recommend we
> > use adopt and require the use of the _Atomic(T) macro to disambiguate it
> > also has the advantage of not being churned later when we can do c++23.
> >
> > // using macro
> > typedef struct rte_mcslock {
> > _Atomic(struct rte_mcslock *) next;
>
> This makes it an atomic pointer. Your example above tried making the struct
> rts_mcslock atomic. Probably what you wanted was:
> typedef struct rte_mcslock {
> struct rte_mcslock * _Atomic next;
> ...
> };
this is what my v2 in the patch had. but following your const example
you indicated you preferred the equivalent of `const T' over `T const` i
was trying to illustrate that if you replace T = struct foo * the
compiler can't disambiguate between type and pointer to type and
produces an error.
>
> Like "const", the convention should be putting it before any type, but after
> the "*" for pointers.
i see, thank you for this clarification. I had not understood that you
were suggesting that for pointer types specifically i should use one
placement and for non-pointer types i should use another.
>
> I suppose clang doesn't accept applying _Atomic to incomplete types,
> regardless where you put it... I.e. this should also fail, I guess:
> typedef struct rte_mcslock {
> struct rte_mcslock _Atomic * next;
> ...
> };
actually I think for C11 atomics i think you can actually do this
because you can declare an entire struct object to be atomic. However,
since we need to intersect with what non-C11 gcc builtin atomics do we
would not be able to make struct objects atomic as gcc only let's you do
atomic things with integer and pointer types.
>
> > ...
> > };
> >
> > this is much easier at a glance to know when the specified type is the T
> > or the T * similarly in parameter lists it becomes more clear too.
> >
> > e.g.
> > void foo(int *v)
> >
> > that it is either void foo(_Atomic(int) *v) or void foo(_Atomic(int *)
> > v) becomes
> > much clearer without having to do mental gymnastics.
>
> The same could be said about making "const" clearer:
> void foo(const(int) * v) instead of void foo(const int * v), and
> void foo(const(int *) v) instead of void foo(int * const v).
>
> Luckily, we don't need toolchain specific handling of "const", so let's just
> leave that the way it is. :-)
>
> >
> > so i propose we retain
> >
> > #define __rte_atomic _Atomic
> >
> > allow it to be used in contexts where we need a type-qualifier.
> > note:
> > most of the cases where _Atomic is used as a type-qualifier it
> > is a red flag that we are sensitive to an implementation detail
> > of the compiler. in time i hope most of these will go away as we
> > remove deprecated rte_atomic_xx apis.
> >
> > but also introduce the following macro
> >
> > #define RTE_ATOMIC(type) _Atomic(type)
> > require it be used in the contexts that we are using it as a type-
> > specifier.
> >
> > if folks agree with this please reply back positively and i'll update
> > the series. feel free to propose alternate names or whatever, but sooner
> > than later so i don't have to churn things too much :)
>
> +1 to Tyler's updated proposal, with macro names as suggested.
yeah, I think it really helps clarify the pointer vs regular type
specification by whacking the ( ) around what we are talking about
instead of using positioning of _Atomic in two different places.
>
> If anyone disagrees, please speak up soon!
>
> If in doubt, please read https://en.cppreference.com/w/c/language/atomic
> carefully. It says:
> (1) _Atomic(type-name) (since C11): Use as a type specifier; this designates
> a new atomic type.
> (2) _Atomic type-name (since C11): Use as a type qualifier; this designates
> the atomic version of type-name. In this role, it may be mixed with const,
> volatile, and restrict, although unlike other qualifiers, the atomic version
> of type-name may have a different size, alignment, and object representation.
>
> NB: I hadn't noticed this before, otherwise I had probably suggested using
> _Atomic(T) earlier on. We learn something new every day. :-)
yeah, i knew about this which is why i was being really careful about
'qualification' vs 'specification' in my mails.
>
> >
> > thanks!
>
> Sorry about the late response, Tyler. Other work prevented me from setting
> aside coherent time to review your updated proposal.
meh it's okay, based on the other thread i kind of guessed you might
agree with using _Atomic(T) so i just submitted a new version an hour
ago with the changes. i hope it meets your approval, one thing i'm kind
of edgy about is the actual macro name itself RTE_ATOMIC(type) it seems
kinda ugly, so if someone has an opinion there i'm open to it.
>
> >
> > >
> > > The DPDK coding style guidelines doesn't mention where to place
> > "const", but looking at the code, it seems to use "const unsigned int"
> > and "const char *".
> >
> > we probably should document it as a convention and most likely we should
> > adopt what is already in use more commonly.
>
> +1, but not as part of this series. :-)
i'll look into doing it once we get this series merged.
thanks!
