jump_label: Provide hotplug context variants

As using the normal static key API under the hotplug lock is
pretty much impossible, let's provide a variant of some of them
that require the hotplug lock to have already been taken.

These function are only meant to be used in CPU hotplug callbacks.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Leo Yan <leo.yan@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/20170801080257.5056-4-marc.zyngier@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

3 files changed, 42 insertions, 6 deletions

diff --git a/Documentation/static-keys.txt b/Documentation/static-keys.txt
index 870b4be3cb11..ab16efe0c79d 100644
--- a/Documentation/static-keys.txt
+++ b/Documentation/static-keys.txt
@@ -154,6 +154,21 @@ and 'static_key_count()'.  In general, if you use these functions, they
 should be protected with the same mutex used around the enable/disable
 or increment/decrement function.
 
+Note that switching branches results in some locks being taken,
+particularly the CPU hotplug lock (in order to avoid races against
+CPUs being brought in the kernel whilst the kernel is getting
+patched). Calling the static key API from within a hotplug notifier is
+thus a sure deadlock recipe. In order to still allow use of the
+functionnality, the following functions are provided:
+
+        static_key_enable_cpuslocked()
+        static_key_disable_cpuslocked()
+        static_branch_enable_cpuslocked()
+        static_branch_disable_cpuslocked()
+
+These functions are *not* general purpose, and must only be used when
+you really know that you're in the above context, and no other.
+
 Where an array of keys is required, it can be defined as::
 
         DEFINE_STATIC_KEY_ARRAY_TRUE(keys, count);
diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
index 740a42ea7f7f..cd5861651b17 100644
--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -163,6 +163,8 @@ extern void jump_label_apply_nops(struct module *mod);
 extern int static_key_count(struct static_key *key);
 extern void static_key_enable(struct static_key *key);
 extern void static_key_disable(struct static_key *key);
+extern void static_key_enable_cpuslocked(struct static_key *key);
+extern void static_key_disable_cpuslocked(struct static_key *key);
 
 /*
  * We should be using ATOMIC_INIT() for initializing .enabled, but
@@ -254,6 +256,9 @@ static inline void static_key_disable(struct static_key *key)
         atomic_set(&key->enabled, 0);
 }
 
+#define static_key_enable_cpuslocked(k)         static_key_enable((k))
+#define static_key_disable_cpuslocked(k)        static_key_disable((k))
+
 #define STATIC_KEY_INIT_TRUE    { .enabled = ATOMIC_INIT(1) }
 #define STATIC_KEY_INIT_FALSE   { .enabled = ATOMIC_INIT(0) }
 
@@ -415,8 +420,10 @@ extern bool ____wrong_branch_error(void);
  * Normal usage; boolean enable/disable.
  */
 
-#define static_branch_enable(x)         static_key_enable(&(x)->key)
-#define static_branch_disable(x)        static_key_disable(&(x)->key)
+#define static_branch_enable(x)                 static_key_enable(&(x)->key)
+#define static_branch_disable(x)                static_key_disable(&(x)->key)
+#define static_branch_enable_cpuslocked(x)      static_key_enable_cpuslocked(&(x)->key)
+#define static_branch_disable_cpuslocked(x)     static_key_disable_cpuslocked(&(x)->key)
 
 #endif /* __ASSEMBLY__ */
 
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index cc6d815c75ed..0bf2e8f5244a 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -126,15 +126,15 @@ void static_key_slow_inc(struct static_key *key)
 }
 EXPORT_SYMBOL_GPL(static_key_slow_inc);
 
-void static_key_enable(struct static_key *key)
+void static_key_enable_cpuslocked(struct static_key *key)
 {
         STATIC_KEY_CHECK_USE();
+
         if (atomic_read(&key->enabled) > 0) {
                 WARN_ON_ONCE(atomic_read(&key->enabled) != 1);
                 return;
         }
 
-        cpus_read_lock();
         jump_label_lock();
         if (atomic_read(&key->enabled) == 0) {
                 atomic_set(&key->enabled, -1);
@@ -145,23 +145,37 @@ void static_key_enable(struct static_key *key)
                 atomic_set_release(&key->enabled, 1);
         }
         jump_label_unlock();
+}
+EXPORT_SYMBOL_GPL(static_key_enable_cpuslocked);
+
+void static_key_enable(struct static_key *key)
+{
+        cpus_read_lock();
+        static_key_enable_cpuslocked(key);
         cpus_read_unlock();
 }
 EXPORT_SYMBOL_GPL(static_key_enable);
 
-void static_key_disable(struct static_key *key)
+void static_key_disable_cpuslocked(struct static_key *key)
 {
         STATIC_KEY_CHECK_USE();
+
         if (atomic_read(&key->enabled) != 1) {
                 WARN_ON_ONCE(atomic_read(&key->enabled) != 0);
                 return;
         }
 
-        cpus_read_lock();
         jump_label_lock();
         if (atomic_cmpxchg(&key->enabled, 1, 0))
                 jump_label_update(key);
         jump_label_unlock();
+}
+EXPORT_SYMBOL_GPL(static_key_disable_cpuslocked);
+
+void static_key_disable(struct static_key *key)
+{
+        cpus_read_lock();
+        static_key_disable_cpuslocked(key);
         cpus_read_unlock();
 }
 EXPORT_SYMBOL_GPL(static_key_disable);