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#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 consitent 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/config.h>
#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 { 0, SPIN_LOCK_UNLOCKED }
#define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
/* 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 inline unsigned read_seqbegin(const seqlock_t *sl)
{
unsigned ret = sl->sequence;
smp_rmb();
return ret;
}
/* Test if reader processed invalid data.
* If initial values is odd,
* then writer had already started when section was entered
* If sequence value changed
* then writer changed data while in section
*
* Using xor saves one conditional branch.
*/
static inline int read_seqretry(const seqlock_t *sl, unsigned iv)
{
smp_rmb();
return (iv & 1) | (sl->sequence ^ iv);
}
/*
* 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 */
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