[PATCH] prctl.2: Document new PR_SET_CHILD_SUBREAPER semantics

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

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@virtuozzo.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