From: Steven Rostedt Date: Wed, 13 May 2009 17:52:19 +0000 (-0400) Subject: x86/function-graph: fix constraint for recording old return value X-Git-Tag: v2.6.30-rc7~31^2~1 X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=aa512a27e9e8ed32f31b15eec67ab1ceca33839b;p=pandora-kernel.git x86/function-graph: fix constraint for recording old return value After upgrading from gcc 4.2.2 to 4.4.0, the function graph tracer broke. Investigating, I found that in the asm that replaces the return value, gcc was using the same register for the old value as it was for the new value. mov (addr), old mov new, (addr) But if old and new are the same register, we clobber new with old! I first thought this was a bug in gcc 4.4.0 and reported it: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40132 Andrew Pinski responded (quickly), saying that it was correct gcc behavior and the code needed to denote old as an "early clobber". Instead of "=r"(old), we need "=&r"(old). [Impact: keep function graph tracer from breaking with gcc 4.4.0 ] Signed-off-by: Steven Rostedt --- diff --git a/arch/x86/kernel/ftrace.c b/arch/x86/kernel/ftrace.c index 18dfa30795c9..b79c5533c421 100644 --- a/arch/x86/kernel/ftrace.c +++ b/arch/x86/kernel/ftrace.c @@ -442,7 +442,7 @@ void prepare_ftrace_return(unsigned long *parent, unsigned long self_addr) _ASM_EXTABLE(1b, 4b) _ASM_EXTABLE(2b, 4b) - : [old] "=r" (old), [faulted] "=r" (faulted) + : [old] "=&r" (old), [faulted] "=r" (faulted) : [parent] "r" (parent), [return_hooker] "r" (return_hooker) : "memory" );