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

[Christian Brauner <christian at brauner dot io>]

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?

Christian