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

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Date:   Thu, 5 Dec 2019 20:17:59 +0100
From:   Jessica Yu <jeyu@...nel.org>
To:     Masami Hiramatsu <mhiramat@...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>
Subject: Re: [PATCH] modules: lockdep: Suppress suspicious RCU usage warning

+++ Masami Hiramatsu [03/12/19 15:14 +0900]:
>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()

s/module_lock/module_mutex/, but you don't have to respin the patch
just for this.

>to suppress this warning, This also fixes similar issue in
>mod_find() and each_symbol_section().
>
>Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>

Thanks Masami! This looks good. I'll queue this up shortly after the
merge window.

Jessica

>---
> 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))
>