linux-kernel - Re: [PATCH] prctl.2: Document new PR_SET_CHILD

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Date:   Sat, 28 Jan 2017 11:47:25 +1300
From:   "Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:     Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
Cc:     linux-man <linux-man@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Oleg Nesterov <oleg@...hat.com>,
        Nicolas Pitre <nicolas.pitre@...aro.org>,
        Michal Hocko <mhocko@...e.com>,
        Stanislav Kinsburskiy <skinsbursky@...tuozzo.com>,
        Mateusz Guzik <mguzik@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Konstantin Khorenko <khorenko@...tuozzo.com>,
        Lennart Poettering <lennart@...ttering.net>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

Hello Pavel,

On 27 January 2017 at 23:11, Pavel Tikhomirov <ptikhomirov@...tuozzo.com> wrote:
> old semantics was non deterministic and worked differently
> depending on the external factors, but nothing changes if
> process first sets itself subreaper and only after forks

When did the kernel behavior change?

Cheers,

Michael


> Signed-off-by: Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
> ---
>  man2/prctl.2 | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/man2/prctl.2 b/man2/prctl.2
> index 97cf21a..84fbd7e 100644
> --- a/man2/prctl.2
> +++ b/man2/prctl.2
> @@ -162,20 +162,30 @@ if
>  is zero, unset the attribute.
>
>  When a process is marked as a child subreaper,
> -all of the children that it creates, and their descendants,
> +all of the children that it creates or have created already, and their descendants,
>  will be marked as having a subreaper.
>  In effect, a subreaper fulfills the role of
>  .BR init (1)
>  for its descendant processes.
> -Upon termination of a process
> -that is orphaned (i.e., its immediate parent has already terminated)
> -and marked as having a subreaper,
> -the nearest still living ancestor subreaper
> -will receive a
> +Upon termination of a process having a subreaper,
> +all its children become orphaned
> +and will be reparented to the nearest still living ancestor subreaper.
> +So that on it's adopted child termination
> +these subreaper will receive a
>  .BR SIGCHLD
>  signal and will be able to
>  .BR wait (2)
> -on the process to discover its termination status.
> +on the child to discover its termination status.
> +
> +Note, that on older kernels these prctl works slightly different.
> +Child subreaper process was not actualy the
> +.BR init (1)
> +for all its descendants.
> +If process forks a child while not been a child subreaper,
> +and after sets himself child subreaper,
> +sub-tree of the child might or might not reparent to the subreaper,
> +depending on the configuration of ancestors of the subreaper,
> +at the time of forking our subtree.
>  .TP
>  .BR PR_GET_CHILD_SUBREAPER " (since Linux 3.4)"
>  Return the "child subreaper" setting of the caller,
> --
> 2.9.3
>



-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/