ftrace: do not trace library functions

make function tracing more robust: do not trace library functions.

We've already got a sizable list of exceptions:

 ifdef CONFIG_FTRACE
 # Do not profile string.o, since it may be used in early boot or vdso
 CFLAGS_REMOVE_string.o = -pg
 # Also do not profile any debug utilities
 CFLAGS_REMOVE_spinlock_debug.o = -pg
 CFLAGS_REMOVE_list_debug.o = -pg
 CFLAGS_REMOVE_debugobjects.o = -pg
 CFLAGS_REMOVE_find_next_bit.o = -pg
 CFLAGS_REMOVE_cpumask.o = -pg
 CFLAGS_REMOVE_bitmap.o = -pg
 endif

... and the pattern has been that random library functionality showed
up in ftrace's critical path (outside of its recursion check), causing
hard to debug lockups.

So be a bit defensive about it and exclude all lib/*.o functions by
default. It's not that they are overly interesting for tracing purposes
anyway. Specific ones can still be traced, in an opt-in manner.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

lib/Makefile

@@ -2,24 +2,17 @@
 # Makefile for some libs needed in the kernel.
 #
 
+ifdef CONFIG_FTRACE
+ORIG_CFLAGS := $(KBUILD_CFLAGS)
+KBUILD_CFLAGS = $(subst -pg,,$(ORIG_CFLAGS))
+endif
+
 lib-y := ctype.o string.o vsprintf.o cmdline.o \
 	 rbtree.o radix-tree.o dump_stack.o \
 	 idr.o int_sqrt.o extable.o prio_tree.o \
 	 sha1.o irq_regs.o reciprocal_div.o argv_split.o \
 	 proportions.o prio_heap.o ratelimit.o
 
-ifdef CONFIG_FTRACE
-# Do not profile string.o, since it may be used in early boot or vdso
-CFLAGS_REMOVE_string.o = -pg
-# Also do not profile any debug utilities
-CFLAGS_REMOVE_spinlock_debug.o = -pg
-CFLAGS_REMOVE_list_debug.o = -pg
-CFLAGS_REMOVE_debugobjects.o = -pg
-CFLAGS_REMOVE_find_next_bit.o = -pg
-CFLAGS_REMOVE_cpumask.o = -pg
-CFLAGS_REMOVE_bitmap.o = -pg
-endif
-
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o