kernel: ftrace: export ftrace_sync_ipi

The following ftrace patch for riscv uses a data store to update ftrace
function. Therefore, a romote fence is required to order it against
function_trace_op updates. The mechanism is similar to the fence between
function_trace_op and update_ftrace_func in the generic ftrace, so we
leverage the same ftrace_sync_ipi function.

[ alex: Fix build warning when !CONFIG_DYNAMIC_FTRACE ]

Signed-off-by: Andy Chiu <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexandre Ghiti <[email protected]>
Signed-off-by: Palmer Dabbelt <[email protected]>

Author: AndybnACT

include/linux/ftrace.h

@@ -635,6 +635,8 @@ enum {
 #define ftrace_get_symaddr(fentry_ip) (0)
 #endif
 
+void ftrace_sync_ipi(void *data);
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 void ftrace_arch_code_modify_prepare(void);

kernel/trace/ftrace.c

@@ -188,7 +188,7 @@ static void ftrace_pid_func(unsigned long ip, unsigned long parent_ip,
 	op->saved_func(ip, parent_ip, op, fregs);
 }
 
-static void ftrace_sync_ipi(void *data)
+void ftrace_sync_ipi(void *data)
 {
 	/* Probably not needed, but do it anyway */
 	smp_rmb();