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/* SPDX-License-Identifier: GPL-2.0 */
#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 irq and softirq uses, readers can use u64_stats_fetch_begin_irq() and
* u64_stats_fetch_retry_irq() 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);
* u64_stats_add(&stats->bytes64, len); // non atomic operation
* u64_stats_inc(&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 = u64_stats_read(&stats->bytes64); // non atomic operation
* tpackets = u64_stats_read(&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
};
#if BITS_PER_LONG == 64
#include <asm/local64.h>
typedef struct {
local64_t v;
} u64_stats_t ;
static inline u64 u64_stats_read(const u64_stats_t *p)
{
return local64_read(&p->v);
}
static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
{
local64_add(val, &p->v);
}
static inline void u64_stats_inc(u64_stats_t *p)
{
local64_inc(&p->v);
}
#else
typedef struct {
u64 v;
} u64_stats_t;
static inline u64 u64_stats_read(const u64_stats_t *p)
{
return p->v;
}
static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
{
p->v += val;
}
static inline void u64_stats_inc(u64_stats_t *p)
{
p->v++;
}
#endif
static inline void u64_stats_init(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG == 32 && defined(CONFIG_SMP)
seqcount_init(&syncp->seq);
#endif
}
static inline void u64_stats_update_begin(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
write_seqcount_begin(&syncp->seq);
#endif
}
static inline void u64_stats_update_end(struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
write_seqcount_end(&syncp->seq);
#endif
}
static inline unsigned long
u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
{
unsigned long flags = 0;
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
local_irq_save(flags);
write_seqcount_begin(&syncp->seq);
#endif
return flags;
}
static inline void
u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
unsigned long flags)
{
#if BITS_PER_LONG==32 && defined(CONFIG_SMP)
write_seqcount_end(&syncp->seq);
local_irq_restore(flags);
#endif
}
static inline unsigned int __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
return 0;
#endif
}
static inline unsigned int u64_stats_fetch_begin(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
preempt_disable();
#endif
return __u64_stats_fetch_begin(syncp);
}
static inline bool __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
return false;
#endif
}
static inline bool u64_stats_fetch_retry(const struct u64_stats_sync *syncp,
unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
preempt_enable();
#endif
return __u64_stats_fetch_retry(syncp, start);
}
/*
* In case irq handlers can update u64 counters, readers can use following helpers
* - SMP 32bit arches use seqcount protection, irq safe.
* - UP 32bit must disable irqs.
* - 64bit have no problem atomically reading u64 values, irq safe.
*/
static inline unsigned int u64_stats_fetch_begin_irq(const struct u64_stats_sync *syncp)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
local_irq_disable();
#endif
return __u64_stats_fetch_begin(syncp);
}
static inline bool u64_stats_fetch_retry_irq(const struct u64_stats_sync *syncp,
unsigned int start)
{
#if BITS_PER_LONG==32 && !defined(CONFIG_SMP)
local_irq_enable();
#endif
return __u64_stats_fetch_retry(syncp, start);
}
#endif /* _LINUX_U64_STATS_SYNC_H */
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