linux-kernel - [PATCH] modules: lockdep: Suppress suspicious RCU usage warning

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Message-Id: <157535364480.17342.7937104819926015512.stgit@devnote2>
Date:   Tue,  3 Dec 2019 15:14:04 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Jessica Yu <jeyu@...nel.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...nel.org>,
        Anders Roxell <anders.roxell@...aro.org>, paulmck@...nel.org,
        "Naveen N . Rao" <naveen.n.rao@...ux.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        David Miller <davem@...emloft.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        mhiramat@...nel.org
Subject: [PATCH] modules: lockdep: Suppress suspicious RCU usage warning

While running kprobe module test, find_module_all() caused
a suspicious RCU usage warning.

-----
 =============================
 WARNING: suspicious RCU usage
 5.4.0-next-20191202+ #63 Not tainted
 -----------------------------
 kernel/module.c:619 RCU-list traversed in non-reader section!!

 other info that might help us debug this:


 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by rmmod/642:
  #0: ffffffff8227da80 (module_mutex){+.+.}, at: __x64_sys_delete_module+0x9a/0x230

 stack backtrace:
 CPU: 0 PID: 642 Comm: rmmod Not tainted 5.4.0-next-20191202+ #63
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
 Call Trace:
  dump_stack+0x71/0xa0
  find_module_all+0xc1/0xd0
  __x64_sys_delete_module+0xac/0x230
  ? do_syscall_64+0x12/0x1f0
  do_syscall_64+0x50/0x1f0
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
 RIP: 0033:0x4b6d49
-----

This is because list_for_each_entry_rcu(modules) is called
without rcu_read_lock(). This is safe because the module_mutex
is locked.

Pass lockdep_is_held(&module_lock) to the list_for_each_entry_rcu()
to suppress this warning, This also fixes similar issue in
mod_find() and each_symbol_section().

Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
---
 kernel/module.c |    9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/module.c b/kernel/module.c
index cb6250be6ee9..38e5c6a7451b 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -214,7 +214,8 @@ static struct module *mod_find(unsigned long addr)
 {
 	struct module *mod;
 
-	list_for_each_entry_rcu(mod, &modules, list) {
+	list_for_each_entry_rcu(mod, &modules, list,
+				lockdep_is_held(&module_mutex)) {
 		if (within_module(addr, mod))
 			return mod;
 	}
@@ -448,7 +449,8 @@ bool each_symbol_section(bool (*fn)(const struct symsearch *arr,
 	if (each_symbol_in_section(arr, ARRAY_SIZE(arr), NULL, fn, data))
 		return true;
 
-	list_for_each_entry_rcu(mod, &modules, list) {
+	list_for_each_entry_rcu(mod, &modules, list,
+				lockdep_is_held(&module_mutex)) {
 		struct symsearch arr[] = {
 			{ mod->syms, mod->syms + mod->num_syms, mod->crcs,
 			  NOT_GPL_ONLY, false },
@@ -616,7 +618,8 @@ static struct module *find_module_all(const char *name, size_t len,
 
 	module_assert_mutex_or_preempt();
 
-	list_for_each_entry_rcu(mod, &modules, list) {
+	list_for_each_entry_rcu(mod, &modules, list,
+				lockdep_is_held(&module_mutex)) {
 		if (!even_unformed && mod->state == MODULE_STATE_UNFORMED)
 			continue;
 		if (strlen(mod->name) == len && !memcmp(mod->name, name, len))