On November 15, 2018 7:30:11 AM PST, "Dmitry V. Levin" <ldv@xxxxxxxxxxxx> 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. Arnd: w.r.t. our previous discussion, this would seem to justify going to timespec(64) for these kind of cases. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
