net: Introduce u64_stats_sync infrastructure

To properly implement 64bits network statistics on 32bit or 64bit hosts,
we provide one new type and four methods, to ease conversions.

Stats producer should use following template granted it already got an
exclusive access to counters (include/linux/u64_stats_sync.h contains
some documentation about details)

    u64_stats_update_begin(&stats->syncp);
    stats->bytes64 += len;
    stats->packets64++;
    u64_stats_update_end(&stats->syncp);

While a consumer should use following template to get consistent
snapshot :

    u64 tbytes, tpackets;
    unsigned int start;

    do {
        start = u64_stats_fetch_begin(&stats->syncp);
        tbytes = stats->bytes64;
        tpackets = stats->packets64;
    } while (u64_stats_fetch_retry(&stats->lock, syncp));

Suggested by David Miller, and comments courtesy of Nick Piggin.
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/u64_stats_sync.h

+#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.
+ *
+ * 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->lock, syncp));
+ *
+ *
+ * Example of use in drivers/net/loopback.c, using per_cpu containers,
+ * in BH disabled context.
+ */
+#include <linux/seqlock.h>
+
+#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
+struct u64_stats_sync {
+	seqcount_t	seq;
+};
+
+static void inline u64_stats_update_begin(struct u64_stats_sync *syncp)
+{
+	write_seqcount_begin(&syncp->seq);
+}
+
+static void inline u64_stats_update_end(struct u64_stats_sync *syncp)
+{
+	write_seqcount_end(&syncp->seq);
+}
+
+static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+	return read_seqcount_begin(&syncp->seq);
+}
+
+static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					 unsigned int start)
+{
+	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)
+{
+}
+
+static unsigned int inline u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
+{
+	return 0;
+}
+
+static bool inline u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
+					 unsigned int start)
+{
+	return false;
+}
+#endif
+
+#endif /* _LINUX_U64_STATS_SYNC_H */