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From: Steven J. Magnani on 2 Apr 2010 14:40
The Microblaze dynamic ftrace code assumes a call ordering that is not met
in all scenarios. Specifically, executing a command similar to:

echo 105 > /sys/kernel/debug/tracing/set_ftrace_pid

before any other tracing-related commands results in a kernel panic:

BUG: failure at arch/microblaze/kernel/ftrace.c:198/ftrace_update_ftrace_func()!

Recoding ftrace_update_ftrace_func() to use &ftrace_caller directly eliminates
the need to capture its address elsewhere (and thus rely on a particular call
sequence).

Signed-off-by: Steven J. Magnani <steve(a)digidescorp.com>
---
diff -uprN a/arch/microblaze/kernel/ftrace.c b/arch/microblaze/kernel/ftrace.c
--- a/arch/microblaze/kernel/ftrace.c 2010-04-02 13:10:54.000000000 -0500
+++ b/arch/microblaze/kernel/ftrace.c 2010-04-02 13:13:29.000000000 -0500
@@ -151,13 +151,10 @@ int ftrace_make_nop(struct module *mod,
return ret;
}

-static int ret_addr; /* initialized as 0 by default */
-
/* I believe that first is called ftrace_make_nop before this function */
int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
{
int ret;
- ret_addr = addr; /* saving where the barrier jump is */
pr_debug("%s: addr:0x%x, rec->ip: 0x%x, imm:0x%x\n",
__func__, (unsigned int)addr, (unsigned int)rec->ip, imm);
ret = ftrace_modify_code(rec->ip, imm);
@@ -194,12 +191,9 @@ int ftrace_update_ftrace_func(ftrace_fun
ret = ftrace_modify_code(ip, upper);
ret += ftrace_modify_code(ip + 4, lower);

- /* We just need to remove the rtsd r15, 8 by NOP */
- BUG_ON(!ret_addr);
- if (ret_addr)
- ret += ftrace_modify_code(ret_addr, MICROBLAZE_NOP);
- else
- ret = 1; /* fault */
+ /* We just need to replace the rtsd r15, 8 with NOP */
+ ret += ftrace_modify_code((unsigned long)&ftrace_caller,
+ MICROBLAZE_NOP);

/* All changes are done - lets do caches consistent */
flush_icache();

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