forked from mirrors/linux
3c118547f8
On PREEMPT_RT the seqcount_t for synchronisation is required on 32bit architectures even on UP because the softirq (and the threaded IRQ handler) can be preempted. With the seqcount_t for synchronisation, a reader with higher priority can preempt the writer and then spin endlessly in read_seqcount_begin() while the writer can't make progress. To avoid such a lock up on PREEMPT_RT the writer must disable preemption during the update. There is no need to disable interrupts because no writer is using this API in hard-IRQ context on PREEMPT_RT. Disable preemption on 32bit-RT within the u64_stats write section. Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by: David S. Miller <davem@davemloft.net>
245 lines
6.6 KiB
C
245 lines
6.6 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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#ifndef _LINUX_U64_STATS_SYNC_H
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#define _LINUX_U64_STATS_SYNC_H
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/*
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* Protect against 64-bit values tearing on 32-bit architectures. This is
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* typically used for statistics read/update in different subsystems.
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*
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* Key points :
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*
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* - Use a seqcount on 32-bit SMP, only disable preemption for 32-bit UP.
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* - The whole thing is a no-op on 64-bit architectures.
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*
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* Usage constraints:
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*
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* 1) 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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*
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* 2) Write side must disable preemption, or a seqcount reader can preempt the
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* writer and also spin forever.
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*
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* 3) Write side must use the _irqsave() variant if other writers, or a reader,
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* can be invoked from an IRQ context.
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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 w.r.t. each other (remember point #2: seqcounts are not
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* used for 64bit architectures).
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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.
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*
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* 6) Readers must use both u64_stats_fetch_{begin,retry}_irq() if the stats
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* might be updated from a hardirq or softirq context (remember point #1:
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* seqcounts are not used for UP kernels). 32-bit UP stat readers could read
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* corrupted 64-bit values otherwise.
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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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* u64_stats_add(&stats->bytes64, len); // non atomic operation
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* u64_stats_inc(&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 = u64_stats_read(&stats->bytes64); // non atomic operation
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* tpackets = u64_stats_read(&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) || defined(CONFIG_PREEMPT_RT))
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seqcount_t seq;
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#endif
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};
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#if BITS_PER_LONG == 64
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#include <asm/local64.h>
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typedef struct {
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local64_t v;
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} u64_stats_t ;
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static inline u64 u64_stats_read(const u64_stats_t *p)
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{
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return local64_read(&p->v);
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}
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static inline void u64_stats_set(u64_stats_t *p, u64 val)
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{
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local64_set(&p->v, val);
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}
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static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
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{
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local64_add(val, &p->v);
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}
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static inline void u64_stats_inc(u64_stats_t *p)
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{
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local64_inc(&p->v);
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}
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#else
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typedef struct {
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u64 v;
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} u64_stats_t;
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static inline u64 u64_stats_read(const u64_stats_t *p)
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{
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return p->v;
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}
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static inline void u64_stats_set(u64_stats_t *p, u64 val)
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{
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p->v = val;
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}
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static inline void u64_stats_add(u64_stats_t *p, unsigned long val)
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{
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p->v += val;
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}
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static inline void u64_stats_inc(u64_stats_t *p)
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{
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p->v++;
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}
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#endif
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#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
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#define u64_stats_init(syncp) seqcount_init(&(syncp)->seq)
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#else
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static inline void u64_stats_init(struct u64_stats_sync *syncp)
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{
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}
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#endif
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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) || defined(CONFIG_PREEMPT_RT))
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if (IS_ENABLED(CONFIG_PREEMPT_RT))
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preempt_disable();
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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) || defined(CONFIG_PREEMPT_RT))
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write_seqcount_end(&syncp->seq);
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if (IS_ENABLED(CONFIG_PREEMPT_RT))
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preempt_enable();
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#endif
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}
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static inline unsigned long
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u64_stats_update_begin_irqsave(struct u64_stats_sync *syncp)
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{
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unsigned long flags = 0;
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#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
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if (IS_ENABLED(CONFIG_PREEMPT_RT))
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preempt_disable();
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else
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local_irq_save(flags);
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write_seqcount_begin(&syncp->seq);
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#endif
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return flags;
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}
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static inline void
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u64_stats_update_end_irqrestore(struct u64_stats_sync *syncp,
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unsigned long flags)
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{
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#if BITS_PER_LONG == 32 && (defined(CONFIG_SMP) || defined(CONFIG_PREEMPT_RT))
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write_seqcount_end(&syncp->seq);
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if (IS_ENABLED(CONFIG_PREEMPT_RT))
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preempt_enable();
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else
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local_irq_restore(flags);
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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) || defined(CONFIG_PREEMPT_RT))
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return read_seqcount_begin(&syncp->seq);
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#else
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return 0;
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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) && !defined(CONFIG_PREEMPT_RT))
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preempt_disable();
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#endif
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return __u64_stats_fetch_begin(syncp);
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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) || defined(CONFIG_PREEMPT_RT))
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return read_seqcount_retry(&syncp->seq, start);
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#else
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return false;
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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) && !defined(CONFIG_PREEMPT_RT))
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preempt_enable();
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#endif
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return __u64_stats_fetch_retry(syncp, start);
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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_PREEMPT_RT)
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preempt_disable();
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#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
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local_irq_disable();
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#endif
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return __u64_stats_fetch_begin(syncp);
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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_PREEMPT_RT)
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preempt_enable();
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#elif BITS_PER_LONG == 32 && !defined(CONFIG_SMP)
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local_irq_enable();
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#endif
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return __u64_stats_fetch_retry(syncp, start);
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}
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#endif /* _LINUX_U64_STATS_SYNC_H */
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