From: Tyler Retzlaff <roretzla@linux.microsoft.com>
To: "Morten Brørup" <mb@smartsharesystems.com>
Cc: thomas@monjalon.net, bruce.richardson@intel.com, dev@dpdk.org,
techboard@dpdk.org,
"Honnappa Nagarahalli" <honnappa.nagarahalli@arm.com>,
"Ruifeng Wang" <ruifeng.wang@arm.com>,
"Jerin Jacob" <jerinj@marvell.com>,
"Sunil Kumar Kori" <skori@marvell.com>,
"Mattias Rönnblom" <mattias.ronnblom@ericsson.com>,
"Joyce Kong" <joyce.kong@arm.com>,
"David Christensen" <drc@linux.vnet.ibm.com>,
"Konstantin Ananyev" <konstantin.v.ananyev@yandex.ru>,
"David Hunt" <david.hunt@intel.com>,
"David Marchand" <david.marchand@redhat.com>
Subject: Re: [PATCH v2 2/6] eal: adapt EAL to present rte optional atomics API
Date: Wed, 16 Aug 2023 13:32:30 -0700 [thread overview]
Message-ID: <20230816203230.GA19981@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net> (raw)
In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35D87B0F@smartserver.smartshare.dk>
On Wed, Aug 16, 2023 at 10:13:22PM +0200, Morten Brørup wrote:
> > From: Tyler Retzlaff [mailto:roretzla@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:roretzla@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 <roretzla@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!
next prev parent reply other threads:[~2023-08-16 20:32 UTC|newest]
Thread overview: 82+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-08-11 1:31 [PATCH 0/6] RFC optional rte optional stdatomics API Tyler Retzlaff
2023-08-11 1:31 ` [PATCH 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-08-11 8:56 ` Bruce Richardson
2023-08-11 9:42 ` Morten Brørup
2023-08-11 15:54 ` Tyler Retzlaff
2023-08-14 9:04 ` Morten Brørup
2023-08-11 1:31 ` [PATCH 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-11 1:31 ` [PATCH 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-11 1:31 ` [PATCH 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-11 1:32 ` [PATCH 5/6] bpf: " Tyler Retzlaff
2023-08-11 1:32 ` [PATCH 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-11 8:57 ` Bruce Richardson
2023-08-11 9:51 ` Morten Brørup
2023-08-11 15:56 ` Tyler Retzlaff
2023-08-14 6:37 ` Morten Brørup
2023-08-11 17:32 ` [PATCH v2 0/6] RFC optional rte optional stdatomics API Tyler Retzlaff
2023-08-11 17:32 ` [PATCH v2 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-08-14 7:06 ` Morten Brørup
2023-08-11 17:32 ` [PATCH v2 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-14 8:00 ` Morten Brørup
2023-08-14 17:47 ` Tyler Retzlaff
2023-08-16 20:13 ` Morten Brørup
2023-08-16 20:32 ` Tyler Retzlaff [this message]
2023-08-11 17:32 ` [PATCH v2 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-14 8:05 ` Morten Brørup
2023-08-11 17:32 ` [PATCH v2 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-14 8:07 ` Morten Brørup
2023-08-11 17:32 ` [PATCH v2 5/6] bpf: " Tyler Retzlaff
2023-08-14 8:11 ` Morten Brørup
2023-08-11 17:32 ` [PATCH v2 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-14 8:12 ` Morten Brørup
2023-08-16 19:19 ` [PATCH v3 0/6] RFC optional rte optional stdatomics API Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-08-16 20:55 ` Morten Brørup
2023-08-16 21:04 ` Tyler Retzlaff
2023-08-16 21:08 ` Morten Brørup
2023-08-16 21:10 ` Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 5/6] bpf: " Tyler Retzlaff
2023-08-16 19:19 ` [PATCH v3 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 0/6] RFC optional rte optional stdatomics API Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-08-17 11:45 ` Morten Brørup
2023-08-17 19:09 ` Tyler Retzlaff
2023-08-18 6:55 ` Morten Brørup
2023-08-16 21:38 ` [PATCH v4 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 5/6] bpf: " Tyler Retzlaff
2023-08-16 21:38 ` [PATCH v4 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-17 11:57 ` Morten Brørup
2023-08-17 19:14 ` Tyler Retzlaff
2023-08-18 7:13 ` Morten Brørup
2023-08-22 18:14 ` Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 0/6] optional rte optional stdatomics API Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 5/6] bpf: " Tyler Retzlaff
2023-08-17 21:42 ` [PATCH v5 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-21 22:27 ` [PATCH v5 0/6] optional rte optional stdatomics API Konstantin Ananyev
2023-08-22 21:00 ` [PATCH v6 0/6] rte atomics API for optional stdatomic Tyler Retzlaff
2023-08-22 21:00 ` [PATCH v6 1/6] eal: provide rte stdatomics optional atomics API Tyler Retzlaff
2023-09-28 8:06 ` Thomas Monjalon
2023-09-29 8:04 ` David Marchand
2023-09-29 8:54 ` Morten Brørup
2023-09-29 9:02 ` David Marchand
2023-09-29 9:26 ` Bruce Richardson
2023-09-29 9:34 ` David Marchand
2023-09-29 10:26 ` Thomas Monjalon
2023-09-29 11:38 ` David Marchand
2023-09-29 11:51 ` Thomas Monjalon
2023-08-22 21:00 ` [PATCH v6 2/6] eal: adapt EAL to present rte " Tyler Retzlaff
2023-08-22 21:00 ` [PATCH v6 3/6] eal: add rte atomic qualifier with casts Tyler Retzlaff
2023-08-22 21:00 ` [PATCH v6 4/6] distributor: adapt for EAL optional atomics API changes Tyler Retzlaff
2023-08-22 21:00 ` [PATCH v6 5/6] bpf: " Tyler Retzlaff
2023-08-22 21:00 ` [PATCH v6 6/6] devtools: forbid new direct use of GCC atomic builtins Tyler Retzlaff
2023-08-29 15:57 ` [PATCH v6 0/6] rte atomics API for optional stdatomic Tyler Retzlaff
2023-09-29 14:09 ` David Marchand
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