WebSVN - shark - Blame - Rev 1618 - /shark/branches/xen/drivers/linuxc26/include/linux/seqlock.h

Rev	Author	Line No.	Line
422	giacomo	1	#ifndef __LINUX_SEQLOCK_H
		2	#define __LINUX_SEQLOCK_H
		3	/*
		4	* Reader/writer consistent mechanism without starving writers. This type of
		5	* lock for data where the reader wants a consitent set of information
		6	* and is willing to retry if the information changes. Readers never
		7	* block but they may have to retry if a writer is in
		8	* progress. Writers do not wait for readers.
		9	*
		10	* This is not as cache friendly as brlock. Also, this will not work
		11	* for data that contains pointers, because any writer could
		12	* invalidate a pointer that a reader was following.
		13	*
		14	* Expected reader usage:
		15	* do {
		16	* seq = read_seqbegin(&foo);
		17	* ...
		18	* } while (read_seqretry(&foo, seq));
		19	*
		20	*
		21	* On non-SMP the spin locks disappear but the writer still needs
		22	* to increment the sequence variables because an interrupt routine could
		23	* change the state of the data.
		24	*
		25	* Based on x86_64 vsyscall gettimeofday
		26	* by Keith Owens and Andrea Arcangeli
		27	*/
		28
		29	#include <linux/config.h>
		30	#include <linux/spinlock.h>
		31	#include <linux/preempt.h>
		32
		33	typedef struct {
		34	unsigned sequence;
		35	spinlock_t lock;
		36	} seqlock_t;
		37
		38	/*
		39	* These macros triggered gcc-3.x compile-time problems. We think these are
		40	* OK now. Be cautious.
		41	*/
		42	#define SEQLOCK_UNLOCKED { 0, SPIN_LOCK_UNLOCKED }
		43	#define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
		44
		45
		46	/* Lock out other writers and update the count.
		47	* Acts like a normal spin_lock/unlock.
		48	* Don't need preempt_disable() because that is in the spin_lock already.
		49	*/
		50	static inline void write_seqlock(seqlock_t *sl)
		51	{
		52	spin_lock(&sl->lock);
		53	++sl->sequence;
		54	smp_wmb();
		55	}
		56
		57	static inline void write_sequnlock(seqlock_t *sl)
		58	{
		59	smp_wmb();
		60	sl->sequence++;
		61	spin_unlock(&sl->lock);
		62	}
		63
		64	static inline int write_tryseqlock(seqlock_t *sl)
		65	{
		66	int ret = spin_trylock(&sl->lock);
		67
		68	if (ret) {
		69	++sl->sequence;
		70	smp_wmb();
		71	}
		72	return ret;
		73	}
		74
		75	/* Start of read calculation -- fetch last complete writer token */
		76	static inline unsigned read_seqbegin(const seqlock_t *sl)
		77	{
		78	unsigned ret = sl->sequence;
		79	smp_rmb();
		80	return ret;
		81	}
		82
		83	/* Test if reader processed invalid data.
		84	* If initial values is odd,
		85	* then writer had already started when section was entered
		86	* If sequence value changed
		87	* then writer changed data while in section
		88	*
		89	* Using xor saves one conditional branch.
		90	*/
		91	static inline int read_seqretry(const seqlock_t *sl, unsigned iv)
		92	{
		93	smp_rmb();
		94	return (iv & 1) \| (sl->sequence ^ iv);
		95	}
		96
		97
		98	/*
		99	* Version using sequence counter only.
		100	* This can be used when code has its own mutex protecting the
		101	* updating starting before the write_seqcountbeqin() and ending
		102	* after the write_seqcount_end().
		103	*/
		104
		105	typedef struct seqcount {
		106	unsigned sequence;
		107	} seqcount_t;
		108
		109	#define SEQCNT_ZERO { 0 }
		110	#define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
		111
		112	/* Start of read using pointer to a sequence counter only. */
		113	static inline unsigned read_seqcount_begin(const seqcount_t *s)
		114	{
		115	unsigned ret = s->sequence;
		116	smp_rmb();
		117	return ret;
		118	}
		119
		120	/* Test if reader processed invalid data.
		121	* Equivalent to: iv is odd or sequence number has changed.
		122	* (iv & 1) \|\| (*s != iv)
		123	* Using xor saves one conditional branch.
		124	*/
		125	static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
		126	{
		127	smp_rmb();
		128	return (iv & 1) \| (s->sequence ^ iv);
		129	}
		130
		131
		132	/*
		133	* Sequence counter only version assumes that callers are using their
		134	* own mutexing.
		135	*/
		136	static inline void write_seqcount_begin(seqcount_t *s)
		137	{
		138	s->sequence++;
		139	smp_wmb();
		140	}
		141
		142	static inline void write_seqcount_end(seqcount_t *s)
		143	{
		144	smp_wmb();
		145	s->sequence++;
		146	}
		147
		148	/*
		149	* Possible sw/hw IRQ protected versions of the interfaces.
		150	*/
		151	#define write_seqlock_irqsave(lock, flags) \
		152	do { local_irq_save(flags); write_seqlock(lock); } while (0)
		153	#define write_seqlock_irq(lock) \
		154	do { local_irq_disable(); write_seqlock(lock); } while (0)
		155	#define write_seqlock_bh(lock) \
		156	do { local_bh_disable(); write_seqlock(lock); } while (0)
		157
		158	#define write_sequnlock_irqrestore(lock, flags) \
		159	do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
		160	#define write_sequnlock_irq(lock) \
		161	do { write_sequnlock(lock); local_irq_enable(); } while(0)
		162	#define write_sequnlock_bh(lock) \
		163	do { write_sequnlock(lock); local_bh_enable(); } while(0)
		164
		165	#define read_seqbegin_irqsave(lock, flags) \
		166	({ local_irq_save(flags); read_seqbegin(lock); })
		167
		168	#define read_seqretry_irqrestore(lock, iv, flags) \
		169	({ \
		170	int ret = read_seqretry(lock, iv); \
		171	local_irq_restore(flags); \
		172	ret; \
		173	})
		174
		175	#endif /* __LINUX_SEQLOCK_H */

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