linux-kernel - [PATCH 1/4] x86/alternatives: free smp_alt

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Date:   Sat, 28 Oct 2017 20:50:54 +0800
From:   Zhou Chengming <zhouchengming1@...wei.com>
To:     <mhiramat@...nel.org>, <bp@...e.de>, <peterz@...radead.org>,
        <ananth@...ux.vnet.ibm.com>, <anil.s.keshavamurthy@...el.com>,
        <davem@...emloft.net>, <tglx@...utronix.de>, <jkosina@...e.cz>,
        <rostedt@...dmis.org>, <mjurczyk@...gle.com>
CC:     <x86@...nel.org>, <linux-kernel@...r.kernel.org>,
        <zhouchengming1@...wei.com>
Subject: [PATCH 1/4] x86/alternatives: free smp_alt_modules when enable smp

In the current code, we don't free smp_alt_modules when enable smp,
so have to wait module unload to call alternatives_smp_module_del()
to free its smp_alt_module. This strategy has shortcomings.

First, maybe some modules don't unload after smp enabled, so these
smp_alt_modules won't be freed even they are useless anymore. Second,
every module has to call alternatives_smp_module_del() when unload
to see if it's in the list even after smp enabled, it's complete
useless work.

And more important, alternatives_smp_module_del() will not traverse
the list after smp enabled, so we don't need a mutex to protect
the list, only need to use preempt_disable().

We can make sure smp_alt_modules will be useless after enable smp,
so free it all. And alternatives_smp_module_del() can return directly
when !uniproc_patched to avoid a list traversal.

Signed-off-by: Zhou Chengming <zhouchengming1@...wei.com>
---
 arch/x86/kernel/alternative.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 3344d33..8549269 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -534,6 +534,9 @@ void __init_or_module alternatives_smp_module_del(struct module *mod)
 	struct smp_alt_module *item;
 
 	mutex_lock(&smp_alt);
+	if (!uniproc_patched)
+		goto unlock;
+
 	list_for_each_entry(item, &smp_alt_modules, next) {
 		if (mod != item->mod)
 			continue;
@@ -541,12 +544,13 @@ void __init_or_module alternatives_smp_module_del(struct module *mod)
 		kfree(item);
 		break;
 	}
+unlock:
 	mutex_unlock(&smp_alt);
 }
 
 void alternatives_enable_smp(void)
 {
-	struct smp_alt_module *mod;
+	struct smp_alt_module *mod, *tmp;
 
 	/* Why bother if there are no other CPUs? */
 	BUG_ON(num_possible_cpus() == 1);
@@ -558,9 +562,12 @@ void alternatives_enable_smp(void)
 		BUG_ON(num_online_cpus() != 1);
 		clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
 		clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
-		list_for_each_entry(mod, &smp_alt_modules, next)
+		list_for_each_entry_safe(mod, tmp, &smp_alt_modules, next) {
 			alternatives_smp_lock(mod->locks, mod->locks_end,
 					      mod->text, mod->text_end);
+			list_del(&mod->next);
+			kfree(mod);
+		}
 		uniproc_patched = false;
 	}
 	mutex_unlock(&smp_alt);
-- 
1.8.3.1