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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.
*
* 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 */
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