kernel: Protect cpu_ent::active_time with sequential lock

atomic_{get, set}64() are problematic on architectures without 64 bit
compare and swap.
Also, using sequential lock instead of atomic access ensures that
any reads from cpu_ent::active_time won't require any writes to shared
memory.

headers/private/kernel/cpu.h

@@ -56,6 +56,7 @@ typedef struct cpu_ent {
 	timer			quantum_timer;
 
 	// keeping track of CPU activity
+	seqlock			active_time_lock;
 	bigtime_t		active_time;
 	bigtime_t		irq_time;
 	bigtime_t		interrupt_time;

src/system/kernel/cpu.cpp

@@ -159,7 +159,15 @@ cpu_get_active_time(int32 cpu)
 	if (cpu < 0 || cpu > smp_get_num_cpus())
 		return 0;
 
-	return atomic_get64(&gCPU[cpu].active_time);
+	bigtime_t activeTime;
+	uint32 count;
+
+	do {
+		count = acquire_read_seqlock(&gCPU[cpu].active_time_lock);
+		activeTime = gCPU[cpu].active_time;
+	} while (!release_read_seqlock(&gCPU[cpu].active_time_lock, count));
+
+	return activeTime;
 }
 
 

src/system/kernel/scheduler/scheduler_cpu.cpp

@@ -180,7 +180,10 @@ CPUEntry::TrackActivity(ThreadData* oldThreadData, ThreadData* nextThreadData)
 			= (oldThread->kernel_time - cpuEntry->last_kernel_time)
 				+ (oldThread->user_time - cpuEntry->last_user_time);
 
-		atomic_add64(&cpuEntry->active_time, active);
+		WriteSequentialLocker locker(cpuEntry->active_time_lock);
+		cpuEntry->active_time += active;
+		locker.Unlock();
+
 		oldThreadData->UpdateActivity(active);
 	}
 

src/system/kernel/smp.cpp

@@ -701,6 +701,7 @@ acquire_read_seqlock(seqlock* lock) {
 
 bool
 release_read_seqlock(seqlock* lock, uint32 count) {
+	arch_cpu_memory_read_barrier();
 	uint32 current = atomic_get((int32*)&lock->count);
 
 	if (count % 2 == 1 || current != count) {