[PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

[PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

[Pavel Tikhomirov]

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

Signed-off-by: Pavel Tikhomirov <ptikhomirov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
---
 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

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Re: [PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

[Michael Kerrisk (man-pages)]

Hello Pavel,

On 27 January 2017 at 23:11, Pavel Tikhomirov <ptikhomirov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> 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-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
> ---
>  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/
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Re: [PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

[Pavel Tikhomirov]

On 01/28/2017 01:47 AM, Michael Kerrisk (man-pages) wrote:
> Hello Pavel,
>
> On 27 January 2017 at 23:11, Pavel Tikhomirov <ptikhomirov-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org> 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-5HdwGun5lf+gSpxsJD1C4w@public.gmane.org>
>> ---
>>  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.
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