[PATCH] RISC-V: Don't check text_mutex during stop_machine

[Palmer Dabbelt]

From: Palmer Dabbelt <palmerdabbelt at google.com>

We're currently using stop_machine() to update ftrace, which means that
the thread that takes text_mutex during ftrace_prepare() may not be the
same as the thread that eventually patches the code.  This isn't
actually a race because the lock is still held (preventing any other
concurrent accesses) and there is only one thread running during
stop_machine(), but it does trigger a lockdep failure.

This patch just elides the lockdep check during stop_machine.

Fixes: c15ac4fd60d5 ("riscv/ftrace: Add dynamic function tracer support")
Suggested-by: Steven Rostedt <rostedt at goodmis.org>
Reported-by: Changbin Du <changbin.du at gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt at google.com>
Signed-off-by: Palmer Dabbelt <palmer at rivosinc.com>

--

Changes since v1 [<20210506071041.417854-1-palmer at dabbelt.com>]:
* Use ftrace_arch_ocde_modify_{prepare,post_process}() to set the flag.
  I remember having a reason I wanted the function when I wrote the v1,
  but it's been almost a year and I forget what that was -- maybe I was
  just crazy, the patch was sent at midnight.
* Fix DYNAMIC_FTRACE=n builds.
---
 arch/riscv/include/asm/ftrace.h |  7 +++++++
 arch/riscv/kernel/ftrace.c      | 12 ++++++++++++
 arch/riscv/kernel/patch.c       | 10 +++++++++-
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
index 04dad3380041..3ac7609f4ee9 100644
--- a/arch/riscv/include/asm/ftrace.h
+++ b/arch/riscv/include/asm/ftrace.h
@@ -81,8 +81,15 @@ do {									\
 struct dyn_ftrace;
 int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
 #define ftrace_init_nop ftrace_init_nop
+extern int riscv_ftrace_in_stop_machine;
 #endif
 
+#else /* CONFIG_DYNAMIC_FTRACE */
+
+#ifndef __ASSEMBLY__
+#define riscv_ftrace_in_stop_machine 0
 #endif
 
+#endif /* CONFIG_DYNAMIC_FTRACE */
+
 #endif /* _ASM_RISCV_FTRACE_H */
diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 4716f4cdc038..c5f77922d7ea 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -11,15 +11,27 @@
 #include <asm/cacheflush.h>
 #include <asm/patch.h>
 
+int riscv_ftrace_in_stop_machine;
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 int ftrace_arch_code_modify_prepare(void) __acquires(&text_mutex)
 {
 	mutex_lock(&text_mutex);
+
+	/*
+	 * The code sequences we use for ftrace can't be patched while the
+	 * kernel is running, so we need to use stop_machine() to modify them
+	 * for now.  This doesn't play nice with text_mutex, we use this flag
+	 * to elide the check.
+	 */
+	riscv_ftrace_in_stop_machine = true;
+
 	return 0;
 }
 
 int ftrace_arch_code_modify_post_process(void) __releases(&text_mutex)
 {
+	riscv_ftrace_in_stop_machine = false;
 	mutex_unlock(&text_mutex);
 	return 0;
 }
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 0b552873a577..7983dba477f0 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -11,6 +11,7 @@
 #include <asm/kprobes.h>
 #include <asm/cacheflush.h>
 #include <asm/fixmap.h>
+#include <asm/ftrace.h>
 #include <asm/patch.h>
 
 struct patch_insn {
@@ -59,8 +60,15 @@ static int patch_insn_write(void *addr, const void *insn, size_t len)
 	 * 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.
+	 *
+	 * We're currently using stop_machine() for ftrace, and while that
+	 * ensures text_mutex is held before installing the mappings it does
+	 * not ensure text_mutex is held by the calling thread.  That's safe
+	 * but triggers a lockdep failure, so just elide it for that specific
+	 * case.
 	 */
-	lockdep_assert_held(&text_mutex);
+	if (!riscv_ftrace_in_stop_machine)
+		lockdep_assert_held(&text_mutex);
 
 	if (across_pages)
 		patch_map(addr + len, FIX_TEXT_POKE1);
-- 
2.34.1