262 lines
6.7 KiB
C
262 lines
6.7 KiB
C
#ifndef __LINUX_SEQLOCK_H
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#define __LINUX_SEQLOCK_H
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/*
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* Reader/writer consistent mechanism without starving writers. This type of
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* lock for data where the reader wants a consistent set of information
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* and is willing to retry if the information changes. Readers never
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* block but they may have to retry if a writer is in
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* progress. Writers do not wait for readers.
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*
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* This is not as cache friendly as brlock. Also, this will not work
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* for data that contains pointers, because any writer could
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* invalidate a pointer that a reader was following.
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*
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* Expected reader usage:
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* do {
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* seq = read_seqbegin(&foo);
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* ...
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* } while (read_seqretry(&foo, seq));
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*
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*
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* On non-SMP the spin locks disappear but the writer still needs
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* to increment the sequence variables because an interrupt routine could
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* change the state of the data.
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*
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* Based on x86_64 vsyscall gettimeofday
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* by Keith Owens and Andrea Arcangeli
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*/
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#include <linux/spinlock.h>
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#include <linux/preempt.h>
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#include <asm/processor.h>
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typedef struct {
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unsigned sequence;
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spinlock_t lock;
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} seqlock_t;
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/*
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* These macros triggered gcc-3.x compile-time problems. We think these are
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* OK now. Be cautious.
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*/
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#define __SEQLOCK_UNLOCKED(lockname) \
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{ 0, __SPIN_LOCK_UNLOCKED(lockname) }
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#define seqlock_init(x) \
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do { \
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(x)->sequence = 0; \
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spin_lock_init(&(x)->lock); \
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} while (0)
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#define DEFINE_SEQLOCK(x) \
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seqlock_t x = __SEQLOCK_UNLOCKED(x)
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/* Lock out other writers and update the count.
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* Acts like a normal spin_lock/unlock.
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* Don't need preempt_disable() because that is in the spin_lock already.
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*/
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static inline void write_seqlock(seqlock_t *sl)
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{
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spin_lock(&sl->lock);
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++sl->sequence;
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smp_wmb();
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}
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static inline void write_sequnlock(seqlock_t *sl)
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{
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smp_wmb();
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sl->sequence++;
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spin_unlock(&sl->lock);
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}
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static inline int write_tryseqlock(seqlock_t *sl)
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{
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int ret = spin_trylock(&sl->lock);
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if (ret) {
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++sl->sequence;
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smp_wmb();
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}
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return ret;
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}
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/* Start of read calculation -- fetch last complete writer token */
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static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
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{
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unsigned ret;
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repeat:
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ret = ACCESS_ONCE(sl->sequence);
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if (unlikely(ret & 1)) {
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cpu_relax();
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goto repeat;
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}
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smp_rmb();
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return ret;
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}
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/*
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* Test if reader processed invalid data.
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*
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* If sequence value changed then writer changed data while in section.
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*/
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static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
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{
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smp_rmb();
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return unlikely(sl->sequence != start);
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}
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/*
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* Version using sequence counter only.
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* This can be used when code has its own mutex protecting the
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* updating starting before the write_seqcountbeqin() and ending
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* after the write_seqcount_end().
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*/
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typedef struct seqcount {
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unsigned sequence;
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} seqcount_t;
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#define SEQCNT_ZERO { 0 }
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#define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
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/**
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* __read_seqcount_begin - begin a seq-read critical section (without barrier)
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*/
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static inline unsigned __read_seqcount_begin(const seqcount_t *s)
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{
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unsigned ret;
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repeat:
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ret = s->sequence;
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if (unlikely(ret & 1)) {
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cpu_relax();
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goto repeat;
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}
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return ret;
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}
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/**
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* read_seqcount_begin - begin a seq-read critical section
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* @s: pointer to seqcount_t
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* Returns: count to be passed to read_seqcount_retry
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*
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* read_seqcount_begin opens a read critical section of the given seqcount.
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* Validity of the critical section is tested by checking read_seqcount_retry
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* function.
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*/
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static inline unsigned read_seqcount_begin(const seqcount_t *s)
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{
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unsigned ret = __read_seqcount_begin(s);
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smp_rmb();
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return ret;
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}
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/**
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* __read_seqcount_retry - end a seq-read critical section (without barrier)
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* @s: pointer to seqcount_t
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* @start: count, from read_seqcount_begin
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* Returns: 1 if retry is required, else 0
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*
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* __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
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* barrier. Callers should ensure that smp_rmb() or equivalent ordering is
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* provided before actually loading any of the variables that are to be
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* protected in this critical section.
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*
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* Use carefully, only in critical code, and comment how the barrier is
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* provided.
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*/
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static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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return unlikely(s->sequence != start);
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}
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/**
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* read_seqcount_retry - end a seq-read critical section
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* @s: pointer to seqcount_t
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* @start: count, from read_seqcount_begin
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* Returns: 1 if retry is required, else 0
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*
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* read_seqcount_retry closes a read critical section of the given seqcount.
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* If the critical section was invalid, it must be ignored (and typically
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* retried).
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*/
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static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
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{
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smp_rmb();
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return __read_seqcount_retry(s, start);
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}
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/*
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* Sequence counter only version assumes that callers are using their
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* own mutexing.
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*/
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static inline void write_seqcount_begin(seqcount_t *s)
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{
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s->sequence++;
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smp_wmb();
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}
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static inline void write_seqcount_end(seqcount_t *s)
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{
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smp_wmb();
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s->sequence++;
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}
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/**
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* write_seqcount_barrier - invalidate in-progress read-side seq operations
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* @s: pointer to seqcount_t
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*
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* After write_seqcount_barrier, no read-side seq operations will complete
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* successfully and see data older than this.
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*/
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static inline void write_seqcount_barrier(seqcount_t *s)
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{
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smp_wmb();
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s->sequence+=2;
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}
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/*
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* Possible sw/hw IRQ protected versions of the interfaces.
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*/
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#define write_seqlock_irqsave(lock, flags) \
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do { local_irq_save(flags); write_seqlock(lock); } while (0)
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#define write_seqlock_irq(lock) \
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do { local_irq_disable(); write_seqlock(lock); } while (0)
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#define write_seqlock_bh(lock) \
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do { local_bh_disable(); write_seqlock(lock); } while (0)
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#define write_sequnlock_irqrestore(lock, flags) \
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do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
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#define write_sequnlock_irq(lock) \
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do { write_sequnlock(lock); local_irq_enable(); } while(0)
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#define write_sequnlock_bh(lock) \
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do { write_sequnlock(lock); local_bh_enable(); } while(0)
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#define read_seqbegin_irqsave(lock, flags) \
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({ local_irq_save(flags); read_seqbegin(lock); })
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#define read_seqretry_irqrestore(lock, iv, flags) \
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({ \
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int ret = read_seqretry(lock, iv); \
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local_irq_restore(flags); \
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ret; \
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})
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#endif /* __LINUX_SEQLOCK_H */
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