- 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>

- Add a comment about interrupts:

6) If counter might be written by an interrupt, readers should block
interrupts.

- Fix a typo in sample of use.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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>