hurd/subhurd: Add use case: debugging the main Hurd system

1 files changed, 25 insertions, 6 deletions

diff --git a/hurd/subhurd.mdwn b/hurd/subhurd.mdwn
index 5b132604..84372dd1 100644
--- a/hurd/subhurd.mdwn
+++ b/hurd/subhurd.mdwn
@@ -1,12 +1,13 @@
-[[!meta copyright="Copyright © 2007, 2008 Free Software Foundation, Inc."]]
+[[!meta copyright="Copyright © 2007, 2008, 2010 Free Software Foundation,
+Inc."]]
 
 [[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
 id="license" text="Permission is granted to copy, distribute and/or modify this
 document under the terms of the GNU Free Documentation License, Version 1.2 or
 any later version published by the Free Software Foundation; with no Invariant
 Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
-is included in the section entitled
-[[GNU Free Documentation License|/fdl]]."]]"""]]
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
 
 A sub-Hurd is like a [[neighbor_Hurd|neighborhurd]], however, makes use of some
 resources provided by another Hurd.  For instance, backing store and the
@@ -17,7 +18,8 @@ attach to them with gdb from the parent
 ([[debugging_via_subhurds|debugging/subhurd]]).  This avoids deadlock, e.g.,
 when the instance of gdb stops the server but requires its use.  (Note: it is
 possible to use [[debugging/gdb/noninvasive_debugging]], but this is less
-flexible.)
+flexible.)  Vice versa, it is also possible to use a subhurd to debug the
+*main* Hurd system, for example, the latter's root file system.
 
 
 # Howto
@@ -105,9 +107,9 @@ inside the subhurd, or to `ssh` directly into the subhurd.
 
 If you want to access the subhurd processes from the outside, e.g. for
 [[debugging_purposes|debugging/subhurd]] (or to get rid of a subhurd that
-didn't exit cleanly...), you need to find out how main Hurd PIDs correspond to
+didn't exit cleanly...), you need to find out how main Hurd [[PID]]s correspond to
 subhurd processes: the subhurd processes appear in the main Hurd (e.g. if doing
-`ps -e`) as unknown processes, and vice versa, but the PIDs are different!  To
+`ps -e`) as unknown processes, and vice versa, but the [[PID]]s are different!  To
 find out which process is which, you can simply compare the order -- while the
 numbers are different, the order should usually match.  Often it also helps to
 look at the number of threads (e.g. using `ps -l`), as many servers have very
@@ -119,3 +121,20 @@ characteristic thread counts.
 Read about using a subhurd for [[debugging_purposes|debugging/subhurd]].
 
 Roland's tutorial about [[running_a_subhurd]].
+
+
+# Use Cases
+
+## Debugging the *Main* Hurd System
+
+A subhurd can be used for debugging the *main* Hurd system.  This works as long
+as the subhurd doesn't use any services provided by the main Hurd.  For
+example, if you already have a subhurd running at the time it happens, you can
+use that one to debug a deadlocked [[translator/ext2fs]] root file system in
+the *main* Hurd.
+
+For this, you need to get a handle to the main Hurd's [[ext2fs
+translator|translator/ext2fs]]'s [[PID]], but this is no problem, as currently
+[[PID]]s are visible across subhurd boundaries.  (It is a [[!taglink
+open_issue_hurd]] whether this is the right thing to do in
+[[open_issues/virtualization]] contexts, but that's how it currently is.)