[PATCH 1/7] static_key: flush rate limit timer on rmmod

["Radim Krčmář" <rkrcmar@redhat.com>]

Fix a bug when we free module memory while timer is pending by marking
deferred static keys and flushing the timer on module unload.

Also make static_key_rate_limit() useable more than once.

Reproducer: (host crasher)
  modprobe kvm_intel
  (sleep 1; echo quit) \
    | qemu-kvm -kernel /dev/null -monitor stdio &
  sleep 0.5
  until modprobe -rv kvm_intel 2>/dev/null; do true; done
  modprobe -v kvm_intel

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
---
Very hacky; I've already queued generalizing ratelimit and applying it
here, but there is still a lot to do on static keys ...

 include/linux/jump_label.h |  1 +
 kernel/jump_label.c        | 17 ++++++++++++++++-
 2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index a507907..848bd15 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -58,6 +58,7 @@ struct static_key {
 #ifdef CONFIG_MODULES
 	struct static_key_mod *next;
 #endif
+	atomic_t deferred;
 };
 
 # include <asm/jump_label.h>
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 297a924..7018042 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -116,8 +116,9 @@ EXPORT_SYMBOL_GPL(static_key_slow_dec_deferred);
 void jump_label_rate_limit(struct static_key_deferred *key,
 		unsigned long rl)
 {
+	if (!atomic_xchg(&key->key.deferred, 1))
+		INIT_DELAYED_WORK(&key->work, jump_label_update_timeout);
 	key->timeout = rl;
-	INIT_DELAYED_WORK(&key->work, jump_label_update_timeout);
 }
 EXPORT_SYMBOL_GPL(jump_label_rate_limit);
 
@@ -185,6 +186,14 @@ static enum jump_label_type jump_label_type(struct static_key *key)
 	return JUMP_LABEL_DISABLE;
 }
 
+static void static_key_rate_limit_flush(struct static_key *key)
+{
+	struct static_key_deferred *dkey =
+		container_of(key, struct static_key_deferred, key);
+	if (atomic_read(&key->deferred))
+		flush_delayed_work(&dkey->work);
+}
+
 void __init jump_label_init(void)
 {
 	struct jump_entry *iter_start = __start___jump_table;
@@ -334,6 +343,12 @@ static void jump_label_del_module(struct module *mod)
 
 		key = (struct static_key *)(unsigned long)iter->key;
 
+		/* We could also check if the static_key is used in its
+		 * defining module and skip this flush then.
+		 * (Rewrite of ratelimit in planned, so we don't care much)
+		 */
+		static_key_rate_limit_flush(key);
+
 		if (__module_address(iter->key) == mod)
 			continue;
 
-- 
1.8.3.1