RISC-V: Don't check text_mutex during stop_machine

We're currently using stop_machine() to update ftrace & kprobes, which
means that the thread that takes text_mutex during 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@goodmis.org>
Reported-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230303143754.4005217-1-conor.dooley@microchip.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

1 files changed, 25 insertions, 3 deletions

diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 8086d1a281cd..575e71d6c8ae 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 {
@@ -20,6 +21,8 @@ struct patch_insn {
 	atomic_t cpu_count;
 };
 
+int riscv_patch_in_stop_machine = false;
+
 #ifdef CONFIG_MMU
 /*
  * The fix_to_virt(, idx) needs a const value (not a dynamic variable of
@@ -60,8 +63,15 @@ 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 & kprobes, 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_patch_in_stop_machine)
+		lockdep_assert_held(&text_mutex);
 
 	if (across_pages)
 		patch_map(addr + len, FIX_TEXT_POKE1);
@@ -125,6 +135,7 @@ NOKPROBE_SYMBOL(patch_text_cb);
 
 int patch_text(void *addr, u32 *insns, int ninsns)
 {
+	int ret;
 	struct patch_insn patch = {
 		.addr = addr,
 		.insns = insns,
@@ -132,7 +143,18 @@ int patch_text(void *addr, u32 *insns, int ninsns)
 		.cpu_count = ATOMIC_INIT(0),
 	};
 
-	return stop_machine_cpuslocked(patch_text_cb,
-				       &patch, cpu_online_mask);
+	/*
+	 * kprobes takes text_mutex, before calling patch_text(), but as we call
+	 * calls stop_machine(), the lockdep assertion in patch_insn_write()
+	 * gets confused by the context in which the lock is taken.
+	 * Instead, ensure the lock is held before calling stop_machine(), and
+	 * set riscv_patch_in_stop_machine to skip the check in
+	 * patch_insn_write().
+	 */
+	lockdep_assert_held(&text_mutex);
+	riscv_patch_in_stop_machine = true;
+	ret = stop_machine_cpuslocked(patch_text_cb, &patch, cpu_online_mask);
+	riscv_patch_in_stop_machine = false;
+	return ret;
 }
 NOKPROBE_SYMBOL(patch_text);