Re: Blocks runtime in the kernel

On Thu, 16 Mar 2023 09:04:29 -0600
Warner Losh <imp@bsdimp.com> wrote:

> On Thu, Mar 16, 2023, 8:06 AM Justin Hibbits <jhibbits@freebsd.org>
> wrote:
> 
> > Most probably know I've been working on the IfAPI conversion of all
> > network drivers in order to hide the contents of `struct ifnet`.
> > I'm pretty much done with the development, and it's all in review.
> > However, there's one bit that I've thought is very clunky since I
> > added it, the if_foreach() iterator function, which iterates over
> > all interfaces in the current VNET, and calls a callback to operate
> > on each interface.  I've noticed that oftentimes I end up with a 2
> > line callback, which just calls if_foreach_addr_type(), so I end up
> > with just trivial callback functions, which seems like a waste.
> >
> > All that backstory to say, would it be beneficial to anyone else to
> > add a (very basic) blocks runtime to the kernel for doing things
> > like this?  The rough change to the IfAPI becomes:
> >
> > int if_foreach_b(int (^)(if_t));
> >
> > __block int foo = 0;
> >
> > if_foreach_b(^(if_t ifp) {
> >   if (if_getlinkstate(ifp) == LINK_STATE_UP)
> >     foo++;
> > });
> >
> > The same could be done for other *_foreach KPIs as well, if this
> > proves out.  I think I could have something working in the next
> > several days.
> >
> > The only technical snag I see with this would be other compilers.
> > I'm not sure if GCC still supports blocks, it did at one point.
> >
> > What do you think?
> >  
> 
> 
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78352
> 
> Suggests that there were issues upstreaming the apple code. So
> there's that.  The gcc12 port I have can't cope with the sample blocks
> code I found on Wikipedia:
> /* blocks-test.c */
> #include <stdio.h>
> #include <Block.h>
> /* Type of block taking nothing returning an int */
> typedef int (^IntBlock)();
> 
> IntBlock MakeCounter(int start, int increment) {
>         __block int i = start;
> 
>         return Block_copy( ^(void) {
>                 int ret = i;
>                 i += increment;
>                 return ret;
>         });
> 
> }
> 
> int main(void) {
>         IntBlock mycounter = MakeCounter(5, 2);
>         printf("First call: %d\n", mycounter());
>         printf("Second call: %d\n", mycounter());
>         printf("Third call: %d\n", mycounter());
> 
>         /* because it was copied, it must also be released */
>         Block_release(mycounter);
> 
>         return 0;
> }
> 
> Our current clang is OK:
> % clang -fblocks a.c -o a -lBlocksRuntime
> %
> 
> But there's no current users of __block in the kernel. There's no
> kernel-specific Block.h file,
> there's no references to BlockRuntime anywhere in the kernel tree and
> the code in
> contrib/llvm-project/compiler-rt/lib/BlocksRuntime is completely
> userland specific. There
> is no kernel support that I could see, since we don't have a
> libkern/OSAtomic.h. I'm happy
> to be corrected on this though: I've never tried to use blocks in the
> kernel and this is grep
> level confidence.
> 
> Clang also doesn't enable blocks unless you pass it -fblock, so you'd
> need to change a fair
> portion of the kernel build system to enable that.
> 
> So I'm thinking regardless of whether or not the project should do
> this, you'll have a fair amount
> of choppy waves ahead of you before you could get to the point of
> starting the ifnet work.
> 
> Warner

Hi Warner,

I did a very very simple test to see what is required link-wise for
blocks in kernel.  This was done by changing
https://reviews.freebsd.org/D38962 to use a block instead of a callback
for the "bootpc_init_count_if_cb".  I didn't include Block.h or
anything, and simply added "-fblocks" to kern.mk.  The result is it
compiles fine, then fails to link (expected) with the following missing
symbols:

_Block_object_dispose
_Block_object_assign
_NSConcreteStackBlock

Reading through
contrib/llvm-project/compiler-rt/lib/BlocksRuntime/runtime.c these
missing symbols look straightforward to implement for the basic case.
I'm not thinking of working within the Clang runtime.c, I'm thinking of
reimplementing the needed functions for this constrained use case (no
need for GC, etc).

This testing was only marginally more than you did, so I'm probably
missing some things as well for more complex use cases.

I'm guessing from GCC's issues that this is a nonstarter anyway?

- Justin