linux-kernel - [PATCH] riscv: ftrace: no need to acquire text_mutex when executed in stop

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Message-Id: <20220310045454.672097-1-changbin.du@gmail.com>
Date:   Thu, 10 Mar 2022 12:54:54 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Paul Walmsley <paul.walmsley@...ive.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Palmer Dabbelt <palmer@...belt.com>
Cc:     linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Changbin Du <changbin.du@...il.com>
Subject: [PATCH] riscv: ftrace: no need to acquire text_mutex when executed in stop_machine

It's safe to patch text segment in stop_machine. No race is possible here.
Besides, there is a false positive for the lock assertion in
patch_insn_write() since the lock is not held by cpu migration thread.

So we actually don't need our ftrace_arch_code_modify_prepare/post(). And
the lock assertion in patch_insn_write() should be removed to avoid
producing lots of false positive warnings.

Signed-off-by: Changbin Du <changbin.du@...il.com>
---
 arch/riscv/kernel/ftrace.c | 16 ++--------------
 arch/riscv/kernel/patch.c  | 12 +++++-------
 2 files changed, 7 insertions(+), 21 deletions(-)

diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
index 4716f4cdc038..381ecdae4ea5 100644
--- a/arch/riscv/kernel/ftrace.c
+++ b/arch/riscv/kernel/ftrace.c
@@ -12,18 +12,6 @@
 #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)
 {
@@ -136,9 +124,9 @@ int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec)
 {
 	int out;
 
-	ftrace_arch_code_modify_prepare();
+	mutex_lock(&text_mutex);
 	out = ftrace_make_nop(mod, rec, MCOUNT_ADDR);
-	ftrace_arch_code_modify_post_process();
+	mutex_unlock(&text_mutex);
 
 	return out;
 }
diff --git a/arch/riscv/kernel/patch.c b/arch/riscv/kernel/patch.c
index 0b552873a577..b7c5dea70924 100644
--- a/arch/riscv/kernel/patch.c
+++ b/arch/riscv/kernel/patch.c
@@ -49,19 +49,17 @@ static void patch_unmap(int fixmap)
 }
 NOKPROBE_SYMBOL(patch_unmap);
 
+/*
+ * Before reaching here, unless executed by stop_machine, it was
+ * expected to lock the text_mutex already which could ensure that
+ * it was safe between each cores.
+ */
 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;
 	int ret;
 
-	/*
-	 * 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);
 
-- 
2.25.1