net: u64_stats_fetch_begin_bh() and u64_stats_fetch_retry_bh()

- Must disable preemption in case of 32bit UP in u64_stats_fetch_begin()
and u64_stats_fetch_retry()

- Add new u64_stats_fetch_begin_bh() and u64_stats_fetch_retry_bh() for
network usage, disabling BH on 32bit UP only.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/u64_stats_sync.h

@@ -27,6 +27,9 @@
  *    (On UP, there is no seqcount_t protection, a reader allowing interrupts could
  *     read partial values)
  *
+ * 7) For softirq uses, readers can use u64_stats_fetch_begin_bh() and
+ *    u64_stats_fetch_retry_bh() helpers
+ *
  * Usage :
  *
  * Stats producer (writer) should use following template granted it already got
@@ -58,54 +61,80 @@
  */
 #include <linux/seqlock.h>
 
-#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 struct u64_stats_sync {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	seqcount_t	seq;
+#endif
 };
 
 static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	write_seqcount_begin(&syncp->seq);
+#endif
 }
 
 static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	write_seqcount_end(&syncp->seq);
+#endif
 }
 
 static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	return read_seqcount_begin(&syncp->seq);
+#else
+#if BITS_PER_LONG==32
+	preempt_disable();
+#endif
+	return 0;
+#endif
 }
 
 static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
 	return read_seqcount_retry(&syncp->seq, start);
-}
-
 #else
-struct u64_stats_sync {
-};
-
-static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
-{
-}
-
-static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
-{
+#if BITS_PER_LONG==32
+	preempt_enable();
+#endif
+	return false;
+#endif
 }
 
-static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+/*
+ * In case softirq handlers can update u64 counters, readers can use following helpers
+ * - SMP 32bit arches use seqcount protection, irq safe.
+ * - UP 32bit must disable BH.
+ * - 64bit have no problem atomically reading u64 values, irq safe.
+ */
+static unsigned int inline u64_stats_fetch_begin_bh(const struct u64_stats_sync *syncp)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	return read_seqcount_begin(&syncp->seq);
+#else
+#if BITS_PER_LONG==32
+	local_bh_disable();
+#endif
 	return 0;
+#endif
 }
 
-static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+static bool inline u64_stats_fetch_retry_bh(const struct u64_stats_sync *syncp,
 					 unsigned int start)
 {
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+	return read_seqcount_retry(&syncp->seq, start);
+#else
+#if BITS_PER_LONG==32
+	local_bh_enable();
+#endif
 	return false;
-}
 #endif
+}
 
 #endif /* _LINUX_U64_STATS_SYNC_H */