net: Introduce u64_stats_sync infrastructure (16b8a476) · Commits · 戴 / test

include/linux/u64_stats_sync.h

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		#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 */

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