sched/vtime: Prevent unstable evaluation of WARN(vtime->state)

As the vtime is sampled under loose seqcount protection by kcpustat, the
vtime fields may change as the code flows. Where logic dictates a field
has a static value, use a READ_ONCE.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Fixes: 74722bb2 ("sched/vtime: Bring up complete kcpustat accessor")
Link: https://lkml.kernel.org/r/20200123180849.28486-1-frederic@kernel.org

kernel/sched/cputime.c

@@ -909,8 +909,10 @@ void task_cputime(struct task_struct *t, u64 *utime, u64 *stime)
 	} while (read_seqcount_retry(&vtime->seqcount, seq));
 }
 
-static int vtime_state_check(struct vtime *vtime, int cpu)
+static int vtime_state_fetch(struct vtime *vtime, int cpu)
 {
+	int state = READ_ONCE(vtime->state);
+
 	/*
 	 * We raced against a context switch, fetch the
 	 * kcpustat task again.
@@ -927,10 +929,10 @@ static int vtime_state_check(struct vtime *vtime, int cpu)
 	 *
 	 * Case 1) is ok but 2) is not. So wait for a safe VTIME state.
 	 */
-	if (vtime->state == VTIME_INACTIVE)
+	if (state == VTIME_INACTIVE)
 		return -EAGAIN;
 
-	return 0;
+	return state;
 }
 
 static u64 kcpustat_user_vtime(struct vtime *vtime)
@@ -949,14 +951,15 @@ static int kcpustat_field_vtime(u64 *cpustat,
 {
 	struct vtime *vtime = &tsk->vtime;
 	unsigned int seq;
-	int err;
 
 	do {
+		int state;
+
 		seq = read_seqcount_begin(&vtime->seqcount);
 
-		err = vtime_state_check(vtime, cpu);
-		if (err < 0)
-			return err;
+		state = vtime_state_fetch(vtime, cpu);
+		if (state < 0)
+			return state;
 
 		*val = cpustat[usage];
 
@@ -969,7 +972,7 @@ static int kcpustat_field_vtime(u64 *cpustat,
 		 */
 		switch (usage) {
 		case CPUTIME_SYSTEM:
-			if (vtime->state == VTIME_SYS)
+			if (state == VTIME_SYS)
 				*val += vtime->stime + vtime_delta(vtime);
 			break;
 		case CPUTIME_USER:
@@ -981,11 +984,11 @@ static int kcpustat_field_vtime(u64 *cpustat,
 				*val += kcpustat_user_vtime(vtime);
 			break;
 		case CPUTIME_GUEST:
-			if (vtime->state == VTIME_GUEST && task_nice(tsk) <= 0)
+			if (state == VTIME_GUEST && task_nice(tsk) <= 0)
 				*val += vtime->gtime + vtime_delta(vtime);
 			break;
 		case CPUTIME_GUEST_NICE:
-			if (vtime->state == VTIME_GUEST && task_nice(tsk) > 0)
+			if (state == VTIME_GUEST && task_nice(tsk) > 0)
 				*val += vtime->gtime + vtime_delta(vtime);
 			break;
 		default:
@@ -1036,23 +1039,23 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
 {
 	struct vtime *vtime = &tsk->vtime;
 	unsigned int seq;
-	int err;
 
 	do {
 		u64 *cpustat;
 		u64 delta;
+		int state;
 
 		seq = read_seqcount_begin(&vtime->seqcount);
 
-		err = vtime_state_check(vtime, cpu);
-		if (err < 0)
-			return err;
+		state = vtime_state_fetch(vtime, cpu);
+		if (state < 0)
+			return state;
 
 		*dst = *src;
 		cpustat = dst->cpustat;
 
 		/* Task is sleeping, dead or idle, nothing to add */
-		if (vtime->state < VTIME_SYS)
+		if (state < VTIME_SYS)
 			continue;
 
 		delta = vtime_delta(vtime);
@@ -1061,15 +1064,15 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
 		 * Task runs either in user (including guest) or kernel space,
 		 * add pending nohz time to the right place.
 		 */
-		if (vtime->state == VTIME_SYS) {
+		if (state == VTIME_SYS) {
 			cpustat[CPUTIME_SYSTEM] += vtime->stime + delta;
-		} else if (vtime->state == VTIME_USER) {
+		} else if (state == VTIME_USER) {
 			if (task_nice(tsk) > 0)
 				cpustat[CPUTIME_NICE] += vtime->utime + delta;
 			else
 				cpustat[CPUTIME_USER] += vtime->utime + delta;
 		} else {
-			WARN_ON_ONCE(vtime->state != VTIME_GUEST);
+			WARN_ON_ONCE(state != VTIME_GUEST);
 			if (task_nice(tsk) > 0) {
 				cpustat[CPUTIME_GUEST_NICE] += vtime->gtime + delta;
 				cpustat[CPUTIME_NICE] += vtime->gtime + delta;
@@ -1080,7 +1083,7 @@ static int kcpustat_cpu_fetch_vtime(struct kernel_cpustat *dst,
 		}
 	} while (read_seqcount_retry(&vtime->seqcount, seq));
 
-	return err;
+	return 0;
 }
 
 void kcpustat_cpu_fetch(struct kernel_cpustat *dst, int cpu)