rcu: don't process callbacks when holding a rcu_read_lock()

Some keyhandlers are calling process_pending_softirqs() while holding
a rcu_read_lock(). This is wrong, as process_pending_softirqs() might
activate rcu calls which should not happen inside a rcu_read_lock().

For that purpose modify process_pending_softirqs() to not allow rcu
callback processing when a rcu_read_lock() is being held.

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

xen/common/softirq.c

@@ -29,6 +29,7 @@ static void __do_softirq(unsigned long ignore_mask)
 {
     unsigned int i, cpu;
     unsigned long pending;
+    bool rcu_allowed = !(ignore_mask & (1ul << RCU_SOFTIRQ));
 
     for ( ; ; )
     {
@@ -38,7 +39,7 @@ static void __do_softirq(unsigned long ignore_mask)
          */
         cpu = smp_processor_id();
 
-        if ( rcu_pending(cpu) )
+        if ( rcu_allowed && rcu_pending(cpu) )
             rcu_check_callbacks(cpu);
 
         if ( ((pending = (softirq_pending(cpu) & ~ignore_mask)) == 0)
@@ -53,9 +54,16 @@ static void __do_softirq(unsigned long ignore_mask)
 
 void process_pending_softirqs(void)
 {
+    unsigned long ignore_mask = (1ul << SCHEDULE_SOFTIRQ) |
+                                (1ul << SCHED_SLAVE_SOFTIRQ);
+
+    /* Block RCU processing in case of rcu_read_lock() held. */
+    if ( preempt_count() )
+        ignore_mask |= 1ul << RCU_SOFTIRQ;
+
     ASSERT(!in_irq() && local_irq_is_enabled());
     /* Do not enter scheduler as it can preempt the calling context. */
-    __do_softirq((1ul << SCHEDULE_SOFTIRQ) | (1ul << SCHED_SLAVE_SOFTIRQ));
+    __do_softirq(ignore_mask);
 }
 
 void do_softirq(void)