rcu: Add automatically released rcu_read_lock variants

RCU_READ_LOCK_GUARD() takes the rcu_read_lock and then uses glib's
g_auto infrastructure (and thus whatever the compiler's hooks are) to
release it on all exits of the block.

WITH_RCU_READ_LOCK_GUARD() is similar but is used as a wrapper for the
lock, i.e.:

   WITH_RCU_READ_LOCK_GUARD() {
       stuff under lock
   }

Note the 'unused' attribute is needed to work around clang bug:
  https://bugs.llvm.org/show_bug.cgi?id=43482

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20191007143642.301445-2-dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

2 files changed, 41 insertions, 0 deletions

diff --git a/docs/devel/rcu.txt b/docs/devel/rcu.txt
index c84e7f42b2..d83fed2f79 100644
--- a/docs/devel/rcu.txt
+++ b/docs/devel/rcu.txt
@@ -187,6 +187,22 @@ The following APIs must be used before RCU is used in a thread:
 Note that these APIs are relatively heavyweight, and should _not_ be
 nested.
 
+Convenience macros
+==================
+
+Two macros are provided that automatically release the read lock at the
+end of the scope.
+
+      RCU_READ_LOCK_GUARD()
+
+         Takes the lock and will release it at the end of the block it's
+         used in.
+
+      WITH_RCU_READ_LOCK_GUARD()  { code }
+
+         Is used at the head of a block to protect the code within the block.
+
+Note that 'goto'ing out of the guarded block will also drop the lock.
 
 DIFFERENCES WITH LINUX
 ======================
diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h
index 22876d1428..9c82683e37 100644
--- a/include/qemu/rcu.h
+++ b/include/qemu/rcu.h
@@ -154,6 +154,31 @@ extern void call_rcu1(struct rcu_head *head, RCUCBFunc *func);
       }),                                                                \
       (RCUCBFunc *)g_free);
 
+typedef void RCUReadAuto;
+static inline RCUReadAuto *rcu_read_auto_lock(void)
+{
+    rcu_read_lock();
+    /* Anything non-NULL causes the cleanup function to be called */
+    return (void *)(uintptr_t)0x1;
+}
+
+static inline void rcu_read_auto_unlock(RCUReadAuto *r)
+{
+    rcu_read_unlock();
+}
+
+G_DEFINE_AUTOPTR_CLEANUP_FUNC(RCUReadAuto, rcu_read_auto_unlock)
+
+#define WITH_RCU_READ_LOCK_GUARD() \
+    WITH_RCU_READ_LOCK_GUARD_(_rcu_read_auto##__COUNTER__)
+
+#define WITH_RCU_READ_LOCK_GUARD_(var) \
+    for (g_autoptr(RCUReadAuto) var = rcu_read_auto_lock(); \
+        (var); rcu_read_auto_unlock(var), (var) = NULL)
+
+#define RCU_READ_LOCK_GUARD() \
+    g_autoptr(RCUReadAuto) _rcu_read_auto __attribute__((unused)) = rcu_read_auto_lock()
+
 #ifdef __cplusplus
 }
 #endif