Re: extending wait4(2) or waitid(2) linux syscall

[Dave Martin <Dave.Martin@arm.com>]

On Thu, Nov 15, 2018 at 06:30:11PM +0300, Dmitry V. Levin wrote:
> On Thu, Nov 15, 2018 at 06:39:03AM -0800, Arnd Bergmann wrote:
> > On Thu, Nov 15, 2018 at 6:05 AM Dmitry V. Levin wrote:
> > > On Thu, Apr 20, 2017 at 03:20:51PM +0200, Albert ARIBAUD wrote:
> [...]
> > > > https://sourceware.org/glibc/wiki/Y2038ProofnessDesign?rev=146
> > > Is there any rationale for marking wait4 as an obsolete API?
> > 
> > In the *kernel* syscall API, wait4(2) is obsoleted by waitid(2), which is
> > a strict superset of its functionality.
> > 
> > In the libc API, this is different, as wait4() does not have a replacement
> > that is exposed to user space directly. I expect glibc to implement
> > wait4() on top of the kernel's waitid().
> > 
> > There has not been a final decision on which variant of waitid() that would
> > be. The easiest option would be to not change it at all: new architectures
> > (rv32, csky, nanomips/p32, ...) would keep exposing the traditional
> > waitid() in Linux, with its 32-bit time_t based rusage structure, but drop the
> > wait4(). glibc then has to convert between the kernel's rusage and the
> > user space rusage indefinitely.
> > 
> > Alternatively, we can create a new version like waitid2() that uses
> > 64-bit time_t in some form, either the exact same rusage that we
> > use on 64-bit architectures and x32, or using a new set of arguments
> > to include further improvements.
> 
> In strace, we have two use cases that require an extended version
> of wait4(2) or waitid(2) syscall.  From your response I understand that
> you'd recommend extending waitid(2) rather than wait4(2), is it correct?
> 
> These two use cases were mentioned in my talk yesterday at LPC 2018,
> here is a brief summary.
> 
> 1. strace needs a race-free invocation of wait4(2) or waitid(2)
> with a different signal mask, this cannot be achieved without
> an extended version of syscall, similar to pselect6(2) extension
> over select(2) and ppoll(2) extension over poll(2).
> 
> Signal mask specification in linux requires two parameters:
> "const sigset_t *sigmask" and "size_t sigsetsize".
> Creating pwait6(2) as an extension of wait4(2) with two arguments
> is straightforward.
> Creating pwaitid(2) as an extension of waitid(2) that already has 5
> arguments would require an indirection similar to pselect6(2).
> 
> 2. The time precision provided by struct rusage returned by wait4(2) and
> waitid(2) is too low for syscall time counting (strace -c) nowadays, this
> can be observing by running in a row a simple command like "strace -c pwd".
> 
> The fix is to return a more appropriate structure than struct rusage
> by the new pwait6(2)/pwaitid(2) syscall mentioned above, where
> struct timeval is replaced with struct timespec or even struct timespec64.

I didn't attend LPC so apologies if I'm missing some background here,
but:

The traditional way would be install a handler for SIGCHLD and
do a sigsuspend()/pselect()/ppoll().  Then when the suspend() returns,
you pump the status of any children with wait*(..., WNOHANG).

Does that not work for you for some reason?

Adding more and more "wait for some stuff" syscalls for every possible
definition of "some stuff" is a slippery slope... IMHO we have way too
many of those already.


One way to improve struct rusage would be to add a new wait flag (say,
WFANCY_RUSAGE) to the existing wait calls to indicate use of a different
struct in place of struct rusage.  It's not clear to me why a brand new
syscall is essential, though obviously that's an option.

Cheers
---Dave