net: stats: Read the statistics in ___gnet_stats_copy_basic() instead of adding.

Since the rework, the statistics code always adds up the byte and packet
value(s). On 32bit architectures a seqcount_t is used in
gnet_stats_basic_sync to ensure that the 64bit values are not modified
during the read since two 32bit loads are required. The usage of a
seqcount_t requires a lock to ensure that only one writer is active at a
time. This lock leads to disabled preemption during the update.

The lack of disabling preemption is now creating a warning as reported
by Naresh since the query done by gnet_stats_copy_basic() is in
preemptible context.

For ___gnet_stats_copy_basic() there is no need to disable preemption
since the update is performed on stack and can't be modified by another
writer. Instead of disabling preemption, to avoid the warning,
simply create a read function to just read the values and return as u64.
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: 67c9e627 ("net: sched: Protect Qdisc::bstats with u64_stats")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/gen_stats.c

@@ -171,20 +171,51 @@ void gnet_stats_add_basic(struct gnet_stats_basic_sync *bstats,
 }
 EXPORT_SYMBOL(gnet_stats_add_basic);
 
+static void gnet_stats_read_basic(u64 *ret_bytes, u64 *ret_packets,
+				  struct gnet_stats_basic_sync __percpu *cpu,
+				  struct gnet_stats_basic_sync *b, bool running)
+{
+	unsigned int start;
+
+	if (cpu) {
+		u64 t_bytes = 0, t_packets = 0;
+		int i;
+
+		for_each_possible_cpu(i) {
+			struct gnet_stats_basic_sync *bcpu = per_cpu_ptr(cpu, i);
+			unsigned int start;
+			u64 bytes, packets;
+
+			do {
+				start = u64_stats_fetch_begin_irq(&bcpu->syncp);
+				bytes = u64_stats_read(&bcpu->bytes);
+				packets = u64_stats_read(&bcpu->packets);
+			} while (u64_stats_fetch_retry_irq(&bcpu->syncp, start));
+
+			t_bytes += bytes;
+			t_packets += packets;
+		}
+		*ret_bytes = t_bytes;
+		*ret_packets = t_packets;
+		return;
+	}
+	do {
+		if (running)
+			start = u64_stats_fetch_begin_irq(&b->syncp);
+		*ret_bytes = u64_stats_read(&b->bytes);
+		*ret_packets = u64_stats_read(&b->packets);
+	} while (running && u64_stats_fetch_retry_irq(&b->syncp, start));
+}
+
 static int
 ___gnet_stats_copy_basic(struct gnet_dump *d,
 			 struct gnet_stats_basic_sync __percpu *cpu,
 			 struct gnet_stats_basic_sync *b,
 			 int type, bool running)
 {
-	struct gnet_stats_basic_sync bstats;
 	u64 bstats_bytes, bstats_packets;
 
-	gnet_stats_basic_sync_init(&bstats);
-	gnet_stats_add_basic(&bstats, cpu, b, running);
-
-	bstats_bytes = u64_stats_read(&bstats.bytes);
-	bstats_packets = u64_stats_read(&bstats.packets);
+	gnet_stats_read_basic(&bstats_bytes, &bstats_packets, cpu, b, running);
 
 	if (d->compat_tc_stats && type == TCA_STATS_BASIC) {
 		d->tc_stats.bytes = bstats_bytes;