1 files changed, 140 insertions, 0 deletions
diff --git a/include/linux/u64_stats_sync.h b/include/linux/u64_stats_sync.h
new file mode 100644
index 000000000000..fa261a0da280
--- /dev/null
+++ b/include/linux/u64_stats_sync.h
@@ -0,0 +1,140 @@
+#ifndef _LINUX_U64_STATS_SYNC_H
+#define _LINUX_U64_STATS_SYNC_H
+
+/*
+ * To properly implement 64bits network statistics on 32bit and 64bit hosts,
+ * we provide a synchronization point, that is a noop on 64bit or UP kernels.
+ *
+ * Key points :
+ * 1) Use a seqcount on SMP 32bits, with low overhead.
+ * 2) Whole thing is a noop on 64bit arches or UP kernels.
+ * 3) Write side must ensure mutual exclusion or one seqcount update could
+ *    be lost, thus blocking readers forever.
+ *    If this synchronization point is not a mutex, but a spinlock or
+ *    spinlock_bh() or disable_bh() :
+ * 3.1) Write side should not sleep.
+ * 3.2) Write side should not allow preemption.
+ * 3.3) If applicable, interrupts should be disabled.
+ *
+ * 4) If reader fetches several counters, there is no guarantee the whole values
+ *    are consistent (remember point 1) : this is a noop on 64bit arches anyway)
+ *
+ * 5) readers are allowed to sleep or be preempted/interrupted : They perform
+ *    pure reads. But if they have to fetch many values, it's better to not allow
+ *    preemptions/interruptions to avoid many retries.
+ *
+ * 6) If counter might be written by an interrupt, readers should block interrupts.
+ *    (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
+ * an exclusive access to counters (a lock is already taken, or per cpu
+ * data is used [in a non preemptable context])
+ *
+ *   spin_lock_bh(...) or other synchronization to get exclusive access
+ *   ...
+ *   u64_stats_update_begin(&stats->syncp);
+ *   stats->bytes64 += len; // non atomic operation
+ *   stats->packets64++;    // non atomic operation
+ *   u64_stats_update_end(&stats->syncp);
+ *
+ * While a consumer (reader) should use following template to get consistent
+ * snapshot for each variable (but no guarantee on several ones)
+ *
+ * u64 tbytes, tpackets;
+ * unsigned int start;
+ *
+ * do {
+ *         start = u64_stats_fetch_begin(&stats->syncp);
+ *         tbytes = stats->bytes64; // non atomic operation
+ *         tpackets = stats->packets64; // non atomic operation
+ * } while (u64_stats_fetch_retry(&stats->syncp, start));
+ *
+ *
+ * Example of use in drivers/net/loopback.c, using per_cpu containers,
+ * in BH disabled context.
+ */
+#include <linux/seqlock.h>
+
+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
+#if BITS_PER_LONG==32
+	preempt_enable();
+#endif
+	return false;
+#endif
+}
+
+/*
+ * 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_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 */