riscv: Use text_mutex instead of patch_lock

We don't need the additional lock protection when patching the text.

There are two patching interfaces here:
 - patch_text: patch code and always synchronize with stop_machine()
 - patch_text_nosync: patch code without synchronization, it's caller's
                      responsibility to synchronize all CPUs if needed.

For the first one, stop_machine() is protected by its own mutex, and
also the irq is already disabled here.

For the second one, in risc-v real case now, it would be used to ftrace
patching the mcount function, since it already running under
kstop_machine(), no other thread will run, so we could use text_mutex
on ftrace side.

Signed-off-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/ftrace.c

@@ -7,10 +7,23 @@
 
 #include <linux/ftrace.h>
 #include <linux/uaccess.h>
+#include <linux/memory.h>
 #include <asm/cacheflush.h>
 #include <asm/patch.h>
 
 #ifdef CONFIG_DYNAMIC_FTRACE
+int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
+{
+	mutex_lock(&text_mutex);
+	return 0;
+}
+
+int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
+{
+	mutex_unlock(&text_mutex);
+	return 0;
+}
+
 static int ftrace_check_current_call(unsigned long hook_pos,
 				     unsigned int *expected)
 {

arch/riscv/kernel/patch.c

@@ -5,6 +5,7 @@
 
 #include <linux/spinlock.h>
 #include <linux/mm.h>
+#include <linux/memory.h>
 #include <linux/uaccess.h>
 #include <linux/stop_machine.h>
 #include <asm/kprobes.h>
@@ -18,8 +19,6 @@ struct patch_insn {
 };
 
 #ifdef CONFIG_MMU
-static DEFINE_RAW_SPINLOCK(patch_lock);
-
 static void *patch_map(void *addr, int fixmap)
 {
 	uintptr_t uintaddr = (uintptr_t) addr;
@@ -49,10 +48,14 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 {
 	void *waddr = addr;
 	bool across_pages = (((uintptr_t) addr & ~PAGE_MASK) + len) > PAGE_SIZE;
-	unsigned long flags = 0;
 	int ret;
 
-	raw_spin_lock_irqsave(&patch_lock, flags);
+	/*
+	 * Before reaching here, it was expected to lock the text_mutex
+	 * already, so we don't need to give another lock here and could
+	 * ensure that it was safe between each cores.
+	 */
+	lockdep_assert_held(&text_mutex);
 
 	if (across_pages)
 		patch_map(addr + len, FIX_TEXT_POKE1);
@@ -66,8 +69,6 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 	if (across_pages)
 		patch_unmap(FIX_TEXT_POKE1);
 
-	raw_spin_unlock_irqrestore(&patch_lock, flags);
-
 	return ret;
 }
 NOKPROBE_SYMBOL(patch_insn_write);