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[pintos-anon] / doc / debug.texi

diff --git a/doc/debug.texi b/doc/debug.texi
index d48e6392e477f7da88f33363b59abba690056da4..7e372ca0a248d7483aa7e6b5de5142b14161fbee 100644 (file)
--- a/doc/debug.texi
+++ b/doc/debug.texi
@@ -11,12 +11,13 @@ introduces you to a few of them.
 * UNUSED NO_RETURN NO_INLINE PRINTF_FORMAT::  
 * Backtraces::                  
 * i386-elf-gdb::                
+* Debugging by Infinite Loop::  
 * Modifying Bochs::             
 * Debugging Tips::              
 @end menu
 
 @node printf
-@section @func{printf}
+@section @code{@code{printf()}}
 
 Don't underestimate the value of @func{printf}.  The way
 @func{printf} is implemented in Pintos, you can call it from
@@ -201,7 +202,15 @@ address.
 (Use a @samp{0x} prefix to specify an address in hex.)
 @end table
 
-If you notice unexplainable behavior while using @command{gdb}, there
+You might notice that @command{gdb} tends to show code being executed
+in an order different from the order in the source.  That is, the
+current statement jumps around seemingly randomly.  This is due to
+GCC's optimizer, which does tend to reorder code.  If it bothers you,
+you can turn off optimization by editing
+@file{pintos/src/Make.config}, removing @option{-O3} from the
+@code{CFLAGS} definition.
+
+If you notice other strange behavior while using @command{gdb}, there
 are three possibilities.  The first is that there is a bug in your
 modified Pintos.  The second is that there is a bug in Bochs's
 interface to @command{gdb} or in @command{gdb} itself.  The third is
@@ -209,6 +218,38 @@ that there is a bug in the original Pintos code.  The first and second
 are quite likely, and you should seriously consider both.  We hope
 that the third is less likely, but it is also possible.
 
+@node Debugging by Infinite Loop
+@section Debugging by Infinite Loop
+
+If you get yourself into a situation where the machine reboots in a
+loop, you've probably hit a ``triple fault.''  In such a situation you
+might not be able to use @func{printf} for debugging, because the
+reboots might be happening even before everything needed for
+@func{printf} is initialized.  In such a situation, you might want to
+try what I call ``debugging by infinite loop.''
+
+What you do is pick a place in the Pintos code, insert the statement
+@code{for (;;);} there, and recompile and run.  There are two likely
+possibilities:
+
+@itemize @bullet
+@item
+The machine hangs without rebooting.  If this happens, you know that
+the infinite loop is running.  That means that whatever caused the
+problem must be @emph{after} the place you inserted the infinite loop.
+Now move the infinite loop later in the code sequence.
+
+@item
+The machine reboots in a loop.  If this happens, you know that the
+machine didn't make it to the infinite loop.  Thus, whatever caused the
+reboot must be @emph{before} the place you inserted the infinite loop.
+Now move the infinite loop earlier in the code sequence.
+@end itemize
+
+If you move around the infinite loop in a ``binary search'' fashion, you
+can use this technique to pin down the exact spot that everything goes
+wrong.  It should only take a few minutes at most.
+
 @node Modifying Bochs
 @section Modifying Bochs