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

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Date:   Sat, 28 Jan 2017 09:38:31 +0300
From:   Pavel Tikhomirov <ptikhomirov@...tuozzo.com>
To:     <mtk.manpages@...il.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

On 01/28/2017 01:47 AM, Michael Kerrisk (man-pages) wrote:
> 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?

Sorry for inconvenience, should have added you to cc of a main patch also.

Kernel behavior haven't changed yet, doc change will be a continuation 
of "[PATCH v2 0/2] prctl: make PR_SET_CHILD_SUBREAPER deterministic", 
see https://lkml.org/lkml/2017/1/27/108

>
> 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
>>
>
>
>

-- 
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.