149 lines
4.3 KiB
C
149 lines
4.3 KiB
C
#ifndef _LINUX_U64_STATS_SYNC_H
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#define _LINUX_U64_STATS_SYNC_H
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/*
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* To properly implement 64bits network statistics on 32bit and 64bit hosts,
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* we provide a synchronization point, that is a noop on 64bit or UP kernels.
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*
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* Key points :
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* 1) Use a seqcount on SMP 32bits, with low overhead.
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* 2) Whole thing is a noop on 64bit arches or UP kernels.
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* 3) Write side must ensure mutual exclusion or one seqcount update could
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* be lost, thus blocking readers forever.
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* If this synchronization point is not a mutex, but a spinlock or
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* spinlock_bh() or disable_bh() :
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* 3.1) Write side should not sleep.
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* 3.2) Write side should not allow preemption.
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* 3.3) If applicable, interrupts should be disabled.
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*
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* 4) If reader fetches several counters, there is no guarantee the whole values
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* are consistent (remember point 1) : this is a noop on 64bit arches anyway)
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*
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* 5) readers are allowed to sleep or be preempted/interrupted : They perform
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* pure reads. But if they have to fetch many values, it's better to not allow
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* preemptions/interruptions to avoid many retries.
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*
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* 6) If counter might be written by an interrupt, readers should block interrupts.
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* (On UP, there is no seqcount_t protection, a reader allowing interrupts could
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* read partial values)
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*
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* 7) For irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
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* u64_stats_fetch_retry_irq() helpers
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*
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* Usage :
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*
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* Stats producer (writer) should use following template granted it already got
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* an exclusive access to counters (a lock is already taken, or per cpu
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* data is used [in a non preemptable context])
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*
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* spin_lock_bh(...) or other synchronization to get exclusive access
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* ...
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* u64_stats_update_begin(&stats->syncp);
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* stats->bytes64 += len; // non atomic operation
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* stats->packets64++; // non atomic operation
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* u64_stats_update_end(&stats->syncp);
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*
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* While a consumer (reader) should use following template to get consistent
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* snapshot for each variable (but no guarantee on several ones)
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*
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* u64 tbytes, tpackets;
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* unsigned int start;
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*
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* do {
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* start = u64_stats_fetch_begin(&stats->syncp);
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* tbytes = stats->bytes64; // non atomic operation
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* tpackets = stats->packets64; // non atomic operation
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* } while (u64_stats_fetch_retry(&stats->syncp, start));
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*
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*
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* Example of use in drivers/net/loopback.c, using per_cpu containers,
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* in BH disabled context.
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*/
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#include <linux/seqlock.h>
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struct u64_stats_sync {
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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seqcount_t seq;
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#endif
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};
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static inline void u64_stats_init(struct u64_stats_sync *syncp)
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{
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#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
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seqcount_init(&syncp->seq);
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#endif
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}
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static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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write_seqcount_begin(&syncp->seq);
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#endif
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}
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static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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write_seqcount_end(&syncp->seq);
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#endif
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}
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static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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return read_seqcount_begin(&syncp->seq);
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#else
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#if BITS_PER_LONG==32
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preempt_disable();
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#endif
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return 0;
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#endif
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}
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static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
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unsigned int start)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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return read_seqcount_retry(&syncp->seq, start);
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#else
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#if BITS_PER_LONG==32
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preempt_enable();
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#endif
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return false;
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#endif
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}
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/*
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* In case irq handlers can update u64 counters, readers can use following helpers
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* - SMP 32bit arches use seqcount protection, irq safe.
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* - UP 32bit must disable irqs.
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* - 64bit have no problem atomically reading u64 values, irq safe.
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*/
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static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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return read_seqcount_begin(&syncp->seq);
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#else
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#if BITS_PER_LONG==32
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local_irq_disable();
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#endif
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return 0;
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#endif
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}
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static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
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unsigned int start)
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{
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#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
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return read_seqcount_retry(&syncp->seq, start);
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#else
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#if BITS_PER_LONG==32
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local_irq_enable();
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#endif
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return false;
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#endif
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}
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#endif /* _LINUX_U64_STATS_SYNC_H */
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