Principles for syscall wrappers, again

[Rich Felker]

On Mon, Jun 01, 2015 at 02:42:56PM +0100, Szabolcs Nagy wrote:
> On 29/05/15 19:14, Torvald Riegel wrote:
> > On Fri, 2015-05-29 at 15:34 +0000, Joseph Myers wrote:
> >> What would be bad is letting discussion of a multiple-function interface 
> >> obstruct consideration and acceptance of a patch adding the basic 
> >> interface.
> > 
> > Sure, but we haven't actually found out whether there would be any
> > obstruction.  Rich says he wouldn't mind having both, others haven't
> > commented.  What's your thought on having both?  Are you opposed to
> > having both, or where do you think the obstruction would come from?
> 
> i think it would be nice if ppl could stop calling syscall directly
> (eg. it can be problematic on ilp32 syscall abi).
> 
> i assume providing the kernel futex api (as a variadic function)
> is the easiest way to get there.
> 
> eg. syscall users in gcc on linux:
> 
> - libstdc++ uses SYS_futex.
> - libgomp uses SYS_futex and SYS_gettid (and it has inline asm to call
> futex syscall directly on various archs).
> - libcilkrts uses SYS_gettid.
> - libitm uses SYS_futex.
> 
> > I mean, in glibc we already do use a different interface than futex(),
> > and always have, right?  And IIRC, futexes have been developed at least
> > with a lot of input from glibc.  Isn't that enough indication that just
> > offering futex() wouldn't be what people actually need?
> 
> some projects already use syscall(SYS_futex,...).
> 
> some of them indeed wrap it as futex_wake/futex_wait but not all.

Another risk of using the names futex_wake/futex_wait is that they may
clash with different wrappers by the same name that an application has
used, making refitting the code to use sys/futex.h versions more work
and more error-prone.

Still I'd rather have both approaches in sys/futex.h than neither...

> debian codesearch finds these futex users:
> 
> android-androresolvd
> aqsis (includes tbb)
> capnproto
> chromium-browser
> ghdl
> groonga
> haproxy
> libxshmfence
> linux-tools
> mariadb-10.0 (includes groonga)
> mumble
> openimageio (includes tbb)
> percona-xtrabackup
> phantomjs (includes qt base)
> qt4-x11 (includes qt base)
> qtbase-opensource-src
> rr
> sbcl
> simgrid
> stress-ng
> tbb

Thanks for doing the research here.

Rich