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/* SPDX-License-Identifier: GPL-2.0 */
#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. There are two types
* of readers:
* 1. Sequence readers which never block a writer but they may have to retry
* if a writer is in progress by detecting change in sequence number.
* Writers do not wait for a sequence reader.
* 2. Locking readers which will wait if a writer or another locking reader
* is in progress. A locking reader in progress will also block a writer
* from going forward. Unlike the regular rwlock, the read lock here is
* exclusive so that only one locking reader can get it.
*
* This is not as cache friendly as brlock. Also, this may not work well
* for data that contains pointers, because any writer could
* invalidate a pointer that a reader was following.
*
* Expected non-blocking 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>
#include <linux/lockdep.h>
#include <linux/compiler.h>
#include <asm/processor.h>
/*
* 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;
#ifdef CONFIG_DEBUG_LOCK_ALLOC
struct lockdep_map dep_map;
#endif
} seqcount_t;
static inline void __seqcount_init(seqcount_t *s, const char *name,
struct lock_class_key *key)
{
/*
* Make sure we are not reinitializing a held lock:
*/
lockdep_init_map(&s->dep_map, name, key, 0);
s->sequence = 0;
}
#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define SEQCOUNT_DEP_MAP_INIT(lockname) \
.dep_map = { .name = #lockname } \
# define seqcount_init(s) \
do { \
static struct lock_class_key __key; \
__seqcount_init((s), #s, &__key); \
} while (0)
static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
{
seqcount_t *l = (seqcount_t *)s;
unsigned long flags;
local_irq_save(flags);
seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
seqcount_release(&l->dep_map, 1, _RET_IP_);
local_irq_restore(flags);
}
#else
# define SEQCOUNT_DEP_MAP_INIT(lockname)
# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
# define seqcount_lockdep_reader_access(x)
#endif
#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
/**
* __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 = READ_ONCE(s->sequence);
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;
}
return ret;
}
/**
* raw_read_seqcount - Read the raw seqcount
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount opens a read critical section of the given
* seqcount without any lockdep checking and without checking or
* masking the LSB. Calling code is responsible for handling that.
*/
static inline unsigned raw_read_seqcount(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
return ret;
}
/**
* raw_read_seqcount_begin - start seq-read critical section w/o lockdep
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
*
* raw_read_seqcount_begin opens a read critical section of the given
* seqcount, but without any lockdep checking. Validity of the critical
* section is tested by checking read_seqcount_retry function.
*/
static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
{
unsigned ret = __read_seqcount_begin(s);
smp_rmb();
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)
{
seqcount_lockdep_reader_access(s);
return raw_read_seqcount_begin(s);
}
/**
* raw_seqcount_begin - begin a seq-read critical section
* @s: pointer to seqcount_t
* Returns: count to be passed to read_seqcount_retry
*
* raw_seqcount_begin opens a read critical section of the given seqcount.
* Validity of the critical section is tested by checking read_seqcount_retry
* function.
*
* Unlike read_seqcount_begin(), this function will not wait for the count
* to stabilize. If a writer is active when we begin, we will fail the
* read_seqcount_retry() instead of stabilizing at the beginning of the
* critical section.
*/
static inline unsigned raw_seqcount_begin(const seqcount_t *s)
{
unsigned ret = READ_ONCE(s->sequence);
smp_rmb();
return ret & ~1;
}
/**
* __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);
}
static inline void raw_write_seqcount_begin(seqcount_t *s)
{
s->sequence++;
smp_wmb();
}
static inline void raw_write_seqcount_end(seqcount_t *s)
{
smp_wmb();
s->sequence++;
}
/**
* raw_write_seqcount_barrier - do a seq write barrier
* @s: pointer to seqcount_t
*
* This can be used to provide an ordering guarantee instead of the
* usual consistency guarantee. It is one wmb cheaper, because we can
* collapse the two back-to-back wmb()s.
*
* Note that, writes surrounding the barrier should be declared atomic (e.g.
* via WRITE_ONCE): a) to ensure the writes become visible to other threads
* atomically, avoiding compiler optimizations; b) to document which writes are
* meant to propagate to the reader critical section. This is necessary because
* neither writes before and after the barrier are enclosed in a seq-writer
* critical section that would ensure readers are aware of ongoing writes.
*
* seqcount_t seq;
* bool X = true, Y = false;
*
* void read(void)
* {
* bool x, y;
*
* do {
* int s = read_seqcount_begin(&seq);
*
* x = X; y = Y;
*
* } while (read_seqcount_retry(&seq, s));
*
* BUG_ON(!x && !y);
* }
*
* void write(void)
* {
* WRITE_ONCE(Y, true);
*
* raw_write_seqcount_barrier(seq);
*
* WRITE_ONCE(X, false);
* }
*/
static inline void raw_write_seqcount_barrier(seqcount_t *s)
{
s->sequence++;
smp_wmb();
s->sequence++;
}
static inline int raw_read_seqcount_latch(seqcount_t *s)
{
/* Pairs with the first smp_wmb() in raw_write_seqcount_latch() */
int seq = READ_ONCE(s->sequence); /* ^^^ */
return seq;
}
/**
* raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t
*
* The latch technique is a multiversion concurrency control method that allows
* queries during non-atomic modifications. If you can guarantee queries never
* interrupt the modification -- e.g. the concurrency is strictly between CPUs
* -- you most likely do not need this.
*
* Where the traditional RCU/lockless data structures rely on atomic
* modifications to ensure queries observe either the old or the new state the
* latch allows the same for non-atomic updates. The trade-off is doubling the
* cost of storage; we have to maintain two copies of the entire data
* structure.
*
* Very simply put: we first modify one copy and then the other. This ensures
* there is always one copy in a stable state, ready to give us an answer.
*
* The basic form is a data structure like:
*
* struct latch_struct {
* seqcount_t seq;
* struct data_struct data[2];
* };
*
* Where a modification, which is assumed to be externally serialized, does the
* following:
*
* void latch_modify(struct latch_struct *latch, ...)
* {
* smp_wmb(); <- Ensure that the last data[1] update is visible
* latch->seq++;
* smp_wmb(); <- Ensure that the seqcount update is visible
*
* modify(latch->data[0], ...);
*
* smp_wmb(); <- Ensure that the data[0] update is visible
* latch->seq++;
* smp_wmb(); <- Ensure that the seqcount update is visible
*
* modify(latch->data[1], ...);
* }
*
* The query will have a form like:
*
* struct entry *latch_query(struct latch_struct *latch, ...)
* {
* struct entry *entry;
* unsigned seq, idx;
*
* do {
* seq = raw_read_seqcount_latch(&latch->seq);
*
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
*
* smp_rmb();
* } while (seq != latch->seq);
*
* return entry;
* }
*
* So during the modification, queries are first redirected to data[1]. Then we
* modify data[0]. When that is complete, we redirect queries back to data[0]
* and we can modify data[1].
*
* NOTE: The non-requirement for atomic modifications does _NOT_ include
* the publishing of new entries in the case where data is a dynamic
* data structure.
*
* An iteration might start in data[0] and get suspended long enough
* to miss an entire modification sequence, once it resumes it might
* observe the new entry.
*
* NOTE: When data is a dynamic data structure; one should use regular RCU
* patterns to manage the lifetimes of the objects within.
*/
static inline void raw_write_seqcount_latch(seqcount_t *s)
{
smp_wmb(); /* prior stores before incrementing "sequence" */
s->sequence++;
smp_wmb(); /* increment "sequence" before following stores */
}
/*
* Sequence counter only version assumes that callers are using their
* own mutexing.
*/
static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
{
raw_write_seqcount_begin(s);
seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
}
static inline void write_seqcount_begin(seqcount_t *s)
{
write_seqcount_begin_nested(s, 0);
}
static inline void write_seqcount_end(seqcount_t *s)
{
seqcount_release(&s->dep_map, 1, _RET_IP_);
raw_write_seqcount_end(s);
}
/**
* write_seqcount_invalidate - invalidate in-progress read-side seq operations
* @s: pointer to seqcount_t
*
* After write_seqcount_invalidate, no read-side seq operations will complete
* successfully and see data older than this.
*/
static inline void write_seqcount_invalidate(seqcount_t *s)
{
smp_wmb();
s->sequence+=2;
}
typedef struct {
struct seqcount seqcount;
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) \
{ \
.seqcount = SEQCNT_ZERO(lockname), \
.lock = __SPIN_LOCK_UNLOCKED(lockname) \
}
#define seqlock_init(x) \
do { \
seqcount_init(&(x)->seqcount); \
spin_lock_init(&(x)->lock); \
} while (0)
#define DEFINE_SEQLOCK(x) \
seqlock_t x = __SEQLOCK_UNLOCKED(x)
/*
* Read side functions for starting and finalizing a read side section.
*/
static inline unsigned read_seqbegin(const seqlock_t *sl)
{
return read_seqcount_begin(&sl->seqcount);
}
static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
{
return read_seqcount_retry(&sl->seqcount, start);
}
/*
* 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);
write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock(&sl->lock);
}
static inline void write_seqlock_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock_bh(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_bh(&sl->lock);
}
static inline void write_seqlock_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
write_seqcount_begin(&sl->seqcount);
}
static inline void write_sequnlock_irq(seqlock_t *sl)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_irq(&sl->lock);
}
static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
{
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
write_seqcount_begin(&sl->seqcount);
return flags;
}
#define write_seqlock_irqsave(lock, flags) \
do { flags = __write_seqlock_irqsave(lock); } while (0)
static inline void
write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
{
write_seqcount_end(&sl->seqcount);
spin_unlock_irqrestore(&sl->lock, flags);
}
/*
* A locking reader exclusively locks out other writers and locking readers,
* but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
* Don't need preempt_disable() because that is in the spin_lock already.
*/
static inline void read_seqlock_excl(seqlock_t *sl)
{
spin_lock(&sl->lock);
}
static inline void read_sequnlock_excl(seqlock_t *sl)
{
spin_unlock(&sl->lock);
}
/**
* read_seqbegin_or_lock - begin a sequence number check or locking block
* @lock: sequence lock
* @seq : sequence number to be checked
*
* First try it once optimistically without taking the lock. If that fails,
* take the lock. The sequence number is also used as a marker for deciding
* whether to be a reader (even) or writer (odd).
* N.B. seq must be initialized to an even number to begin with.
*/
static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
{
if (!(*seq & 1)) /* Even */
*seq = read_seqbegin(lock);
else /* Odd */
read_seqlock_excl(lock);
}
static inline int need_seqretry(seqlock_t *lock, int seq)
{
return !(seq & 1) && read_seqretry(lock, seq);
}
static inline void done_seqretry(seqlock_t *lock, int seq)
{
if (seq & 1)
read_sequnlock_excl(lock);
}
static inline void read_seqlock_excl_bh(seqlock_t *sl)
{
spin_lock_bh(&sl->lock);
}
static inline void read_sequnlock_excl_bh(seqlock_t *sl)
{
spin_unlock_bh(&sl->lock);
}
static inline void read_seqlock_excl_irq(seqlock_t *sl)
{
spin_lock_irq(&sl->lock);
}
static inline void read_sequnlock_excl_irq(seqlock_t *sl)
{
spin_unlock_irq(&sl->lock);
}
static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
{
unsigned long flags;
spin_lock_irqsave(&sl->lock, flags);
return flags;
}
#define read_seqlock_excl_irqsave(lock, flags) \
do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
static inline void
read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
{
spin_unlock_irqrestore(&sl->lock, flags);
}
static inline unsigned long
read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
{
unsigned long flags = 0;
if (!(*seq & 1)) /* Even */
*seq = read_seqbegin(lock);
else /* Odd */
read_seqlock_excl_irqsave(lock, flags);
return flags;
}
static inline void
done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
{
if (seq & 1)
read_sequnlock_excl_irqrestore(lock, flags);
}
#endif /* __LINUX_SEQLOCK_H */
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