Re: [RFC v3 03/23] sysdeps/wait: Use waitid if avaliable

[Christian Brauner <christian at brauner dot io>]

On August 14, 2019 1:11:53 AM GMT+02:00, Rich Felker <dalias@libc.org> wrote:
>On Tue, Aug 13, 2019 at 03:22:02PM -0700, Alistair Francis wrote:
>> On Thu, Jul 25, 2019 at 10:30 AM Christian Brauner
><christian@brauner.io> wrote:
>> >
>> > On July 25, 2019 7:14:05 PM GMT+02:00, ebiederm@xmission.com wrote:
>> > >Arnd Bergmann <arnd@arndb.de> writes:
>> > >
>> > >> On Thu, Jul 25, 2019 at 6:40 AM Rich Felker <dalias@libc.org>
>wrote:
>> > >>> On Wed, Jul 24, 2019 at 05:04:53PM -0700, Alistair Francis
>wrote:
>> > >>> > On Tue, Jul 23, 2019 at 1:45 AM Arnd Bergmann <arnd@arndb.de>
>> > >wrote:
>> > >>> > >
>> > >>> > > Sounds good to me, the debate over what rusage to use
>should not
>> > >hold
>> > >>> > > up the review of the rest of that syscall.
>> > >>> >
>> > >>> > I'm unclear what the final decision is here. What is the
>solution
>> > >are
>> > >>> > we going to have wait4() or add P_PROCESS_PGID to waitid()?
>> > >>> >
>> > >>> > As well as that what is the solution to current
>implementations?
>> > >If we
>> > >>> > add wait4() then there isn't an issue (and I can drop this
>patch)
>> > >but
>> > >>> > if we add P_PROCESS_PGID then we will need a way to handle
>kernels
>> > >>> > with waitid() but no P_PROCESS_PGID. Although my new plan is
>to
>> > >only
>> > >>> > use the waitid syscall if we don't have waitpid or wait4 so
>it
>> > >seems
>> > >>> > like this will only affect RV32 for the time being.
>> > >>>
>> > >>> I would really like some indication which solution will be
>taken,
>> > >>> since it impacts choices that will need to be made in musl very
>> > >soon.
>> > >>> My favorite outcome would be bringing back wait4 for rv32 (and
>> > >>> no-time32 archs in general) *and* adding P_PROCESS_PGID. In the
>> > >short
>> > >>> term, just using wait4 would be the simplest and cleanest for
>us
>> > >(same
>> > >>> as all other archs, no extra case to deal with), but in the
>long
>> > >term
>> > >>> there may be value in having rusage that can represent more
>than 68
>> > >>> cpu-years spent by a process (seems plausible with large
>numbers of
>> > >>> cores).
>> > >>
>> > >> Based on the feedback from Linus and Eric, the most likely
>outcome
>> > >> at the moment seems to be an extension of waitid() to allow
>> > >> P_PGID with id=0 like BSD does, and not bring back wait4() or
>> > >> add P_PROCESS_PGID.
>> > >>
>> > >> So far, I don't think anyone has proposed an actual kernel
>patch.
>> > >> I was hoping that Eric would do it, but I could also send it if
>he's
>> > >> otherwise busy.
>> > >
>> > >So here is what I am looking at.  It still needs to be tested
>> > >and the description needs to be improved so that it properly
>credits
>> > >everyone.  However I think I have the major stroeks correct.
>> > >
>> > >From: "Eric W. Biederman" <ebiederm@xmission.com>
>> > >Date: Tue, 23 Jul 2019 07:44:46 -0500
>> > >Subject: [PATCH] waitid: Add support for waiting for the current
>> > >process group
>> > >
>> > >It was recently discovered that the linux version of waitid is not
>a
>> > >superset of the other wait functions because it does not include
>> > >support for waiting for the current process group.  This has two
>> > >downsides.  An extra system call is needed to get the current
>process
>> > >group, and a signal could come in between the system call that
>> > >retrieved the process gorup and the call to waitid that changes
>the
>> > >current process group.
>> > >
>> > >Allow userspace to avoid both of those issues by defining
>> > >idtype == P_PGID and id == 0 to mean wait for the caller's process
>> > >group at the time of the call.
>> > >
>> > >Arguments can be made for using a different choice of idtype and
>id
>> > >for this case but the BSDs already use this P_PGID and 0 to
>indicate
>> > >waiting for the current process's process group.  So be nice to
>user
>> > >space programmers and don't introduce an unnecessary
>incompatibility.
>> > >
>> > >Some people have noted that the posix description is that
>> > >waitpid will wait for the current process group, and that in
>> > >the presence of pthreads that process group can change.  To get
>> > >clarity on this issue I looked at XNU, FreeBSD, and Luminos.  All
>of
>> > >those flavors of unix waited for the current process group at the
>> > >time of call and as written could not adapt to the process group
>> > >changing after the call.
>> > >
>> > >At one point Linux did adapt to the current process group changing
>but
>> > >that stopped in 161550d74c07 ("pid: sys_wait... fixes").  It has
>been
>> > >over 11 years since Linux has that behavior, no programs that fail
>> > >with the change in behavior have been reported, and I could not
>> > >find any other unix that does this.  So I think it is safe to
>clarify
>> > >the definition of current process group, to current process group
>> > >at the time of the wait function.
>> > >
>> > >Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
>> > >---
>> > > kernel/exit.c | 6 ++++--
>> > > 1 file changed, 4 insertions(+), 2 deletions(-)
>> > >
>> > >diff --git a/kernel/exit.c b/kernel/exit.c
>> > >index a75b6a7f458a..3d86930f035e 100644
>> > >--- a/kernel/exit.c
>> > >+++ b/kernel/exit.c
>> > >@@ -1577,14 +1577,16 @@ static long kernel_waitid(int which, pid_t
>> > >upid, struct waitid_info *infop,
>> > >               break;
>> > >       case P_PGID:
>> > >               type = PIDTYPE_PGID;
>> > >-              if (upid <= 0)
>> > >+              if (upid < 0)
>> > >                       return -EINVAL;
>> > >+              if (upid == 0)
>> > >+                      pid = get_pid(task_pgrp(current));
>> > >               break;
>> > >       default:
>> > >               return -EINVAL;
>> > >       }
>> > >
>> > >-      if (type < PIDTYPE_MAX)
>> > >+      if ((type < PIDTYPE_MAX) && !pid)
>> > >               pid = find_get_pid(upid);
>> > >
>> > >       wo.wo_type      = type;
>> >
>> > Eric, mind if I send this out alongside the P_PIDFD patchset and
>put in a test for it?
>> 
>> Was this ever sent?
>
>It's not upstream. Would be really nice to get this fixed in 5.3 so
>that RV32 will not be missing functionality...
>
>Rich

I'll pick it up as is and send out for review later if I hear no objections.

Christian