From: Ben Pfaff Date: Tue, 20 Dec 2005 21:27:33 +0000 (+0000) Subject: Explain why it sometimes looks like pass() fails. X-Git-Url: https://pintos-os.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=ad15679e64c257df1447acebeed22d1f76607ff6;p=pintos-anon Explain why it sometimes looks like pass() fails. --- diff --git a/doc/debug.texi b/doc/debug.texi index 6432ca3..8fbc7d5 100644 --- a/doc/debug.texi +++ b/doc/debug.texi @@ -119,9 +119,15 @@ Alternatively, it could be that the @file{kernel.o} you passed to the backtrace. Sometimes backtraces can be confusing without implying corruption. -Compiler optimizations can cause surprising behavior. For example, when -a function has called another function as its final action (a @dfn{tail -call}), the calling function may not appear in a backtrace at all. +Compiler optimizations can cause surprising behavior. When a function +has called another function as its final action (a @dfn{tail call}), the +calling function may not appear in a backtrace at all. Similarly, when +function A calls another function B that never returns, the compiler may +optimize such that an unrelated function C appears in the backtrace +instead of A. Function C is simply the function that happens to be in +memory just after A. In the threads project, this is commonly seen in +backtraces for test failures; see @ref{The pass function fails, , +@func{pass} Fails}), for more information. @menu * Backtrace Example:: diff --git a/doc/threads.texi b/doc/threads.texi index 314ee61..1093f33 100644 --- a/doc/threads.texi +++ b/doc/threads.texi @@ -591,6 +591,34 @@ to cause many of the tests to fail. @item How do I run the tests? @xref{Testing}. + +@item Why do I get a test failure in @func{pass}? + +@anchor{The pass function fails} +You are probably looking at a backtrace that looks something like this: + +@example +0xc0108810: debug_panic (../../lib/kernel/debug.c:32) +0xc010a99f: pass (../../tests/threads/tests.c:93) +0xc010bdd3: test_mlfqs_load_1 (../../tests/threads/mlfqs-load-1.c:33) +0xc010a8cf: run_test (../../tests/threads/tests.c:51) +0xc0100452: run_task (../../threads/init.c:283) +0xc0100536: run_actions (../../threads/init.c:333) +0xc01000bb: main (../../threads/init.c:137) +@end example + +This is just confusing output from the @command{backtrace} program. It +does not actually mean that @func{pass} called @func{debug_panic}. In +fact, @func{fail} called @func{debug_panic} (via the @func{PANIC} +macro). GCC knows that @func{debug_panic} does not return, because it +is declared @code{NO_RETURN} (@pxref{Function and Parameter +Attributes}), so it doesn't include any code in @func{pass} to take +control when @func{debug_panic} returns. This means that the return +address on the stack looks like it is at the beginning of the function +that happens to follow @func{fail} in memory, which in this case happens +to be @func{pass}. + +@xref{Backtraces}, for more information. @end table @menu