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