rcu: add assertions to debug build

Xen's RCU implementation relies on no softirq handling taking place
while being in a RCU critical section. Add ASSERT()s in debug builds
in order to catch any violations.

For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
counter additional to preempt_[en|dis]able() as this enables to test
that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
usable there due to __cpu_up() calling process_pending_softirqs()
while holding the cpu hotplug lock).

While at it switch the rcu_read_[un]lock() implementation to static
inline functions instead of macros.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/common/rcupdate.c

@@ -46,6 +46,8 @@
 #include <xen/cpu.h>
 #include <xen/stop_machine.h>
 
+DEFINE_PER_CPU(unsigned int, rcu_lock_cnt);
+
 /* Global control variables for rcupdate callback mechanism. */
 static struct rcu_ctrlblk {
     long cur;           /* Current batch number.                      */

xen/common/softirq.c

@@ -31,6 +31,8 @@ static void __do_softirq(unsigned long ignore_mask)
     unsigned long pending;
     bool rcu_allowed = !(ignore_mask & (1ul << RCU_SOFTIRQ));
 
+    ASSERT(!rcu_allowed || rcu_quiesce_allowed());
+
     for ( ; ; )
     {
         /*
@@ -58,7 +60,7 @@ void process_pending_softirqs(void)
                                 (1ul << SCHED_SLAVE_SOFTIRQ);
 
     /* Block RCU processing in case of rcu_read_lock() held. */
-    if ( preempt_count() )
+    if ( !rcu_quiesce_allowed() )
         ignore_mask |= 1ul << RCU_SOFTIRQ;
 
     ASSERT(!in_irq() && local_irq_is_enabled());

xen/include/xen/rcupdate.h

@@ -32,12 +32,35 @@
 #define __XEN_RCUPDATE_H
 
 #include <xen/cache.h>
+#include <xen/compiler.h>
 #include <xen/spinlock.h>
 #include <xen/cpumask.h>
+#include <xen/percpu.h>
 #include <xen/preempt.h>
 
 #define __rcu
 
+DECLARE_PER_CPU(unsigned int, rcu_lock_cnt);
+
+static inline void rcu_quiesce_disable(void)
+{
+    preempt_disable();
+    this_cpu(rcu_lock_cnt)++;
+    barrier();
+}
+
+static inline void rcu_quiesce_enable(void)
+{
+    barrier();
+    this_cpu(rcu_lock_cnt)--;
+    preempt_enable();
+}
+
+static inline bool rcu_quiesce_allowed(void)
+{
+    return !this_cpu(rcu_lock_cnt);
+}
+
 /**
  * struct rcu_head - callback structure for use with RCU
  * @next: next update requests in a list
@@ -91,16 +114,24 @@ typedef struct _rcu_read_lock rcu_read_lock_t;
  * will be deferred until the outermost RCU read-side critical section
  * completes.
  *
- * It is illegal to block while in an RCU read-side critical section.
+ * It is illegal to process softirqs or block while in an RCU read-side
+ * critical section.
  */
-#define rcu_read_lock(x)       ({ ((void)(x)); preempt_disable(); })
+static inline void rcu_read_lock(rcu_read_lock_t *lock)
+{
+    rcu_quiesce_disable();
+}
 
 /**
  * rcu_read_unlock - marks the end of an RCU read-side critical section.
  *
  * See rcu_read_lock() for more information.
  */
-#define rcu_read_unlock(x)     ({ ((void)(x)); preempt_enable(); })
+static inline void rcu_read_unlock(rcu_read_lock_t *lock)
+{
+    ASSERT(!rcu_quiesce_allowed());
+    rcu_quiesce_enable();
+}
 
 /*
  * So where is rcu_write_lock()?  It does not exist, as there is no