diff options
Diffstat (limited to 'include/linux/seqlock.h')
| -rw-r--r-- | include/linux/seqlock.h | 148 |
1 files changed, 101 insertions, 47 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index d90d8ee29d81..5ce48eab7a2a 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -157,7 +157,7 @@ __seqprop_##lockname##_const_ptr(const seqcount_##lockname##_t *s) \ static __always_inline unsigned \ __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \ { \ - unsigned seq = READ_ONCE(s->seqcount.sequence); \ + unsigned seq = smp_load_acquire(&s->seqcount.sequence); \ \ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \ return seq; \ @@ -170,7 +170,7 @@ __seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \ * Re-read the sequence counter since the (possibly \ * preempted) writer made progress. \ */ \ - seq = READ_ONCE(s->seqcount.sequence); \ + seq = smp_load_acquire(&s->seqcount.sequence); \ } \ \ return seq; \ @@ -208,7 +208,7 @@ static inline const seqcount_t *__seqprop_const_ptr(const seqcount_t *s) static inline unsigned __seqprop_sequence(const seqcount_t *s) { - return READ_ONCE(s->sequence); + return smp_load_acquire(&s->sequence); } static inline bool __seqprop_preemptible(const seqcount_t *s) @@ -263,24 +263,16 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex) #define seqprop_assert(s) __seqprop(s, assert)(s) /** - * __read_seqcount_begin() - begin a seqcount_t read section w/o barrier + * __read_seqcount_begin() - begin a seqcount_t read section * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants * - * __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. - * * Return: count to be passed to read_seqcount_retry() */ #define __read_seqcount_begin(s) \ ({ \ unsigned __seq; \ \ - while ((__seq = seqprop_sequence(s)) & 1) \ + while (unlikely((__seq = seqprop_sequence(s)) & 1)) \ cpu_relax(); \ \ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \ @@ -293,13 +285,7 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex) * * Return: count to be passed to read_seqcount_retry() */ -#define raw_read_seqcount_begin(s) \ -({ \ - unsigned _seq = __read_seqcount_begin(s); \ - \ - smp_rmb(); \ - _seq; \ -}) +#define raw_read_seqcount_begin(s) __read_seqcount_begin(s) /** * read_seqcount_begin() - begin a seqcount_t read critical section @@ -328,12 +314,34 @@ SEQCOUNT_LOCKNAME(mutex, struct mutex, true, mutex) ({ \ unsigned __seq = seqprop_sequence(s); \ \ - smp_rmb(); \ kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); \ __seq; \ }) /** + * raw_seqcount_try_begin() - begin a seqcount_t read critical section + * w/o lockdep and w/o counter stabilization + * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants + * @start: count to be passed to read_seqcount_retry() + * + * Similar to raw_seqcount_begin(), except it enables eliding the critical + * section entirely if odd, instead of doing the speculation knowing it will + * fail. + * + * Useful when counter stabilization is more or less equivalent to taking + * the lock and there is a slowpath that does that. + * + * If true, start will be set to the (even) sequence count read. + * + * Return: true when a read critical section is started. + */ +#define raw_seqcount_try_begin(s, start) \ +({ \ + start = raw_read_seqcount(s); \ + !(start & 1); \ +}) + +/** * raw_seqcount_begin() - begin a seqcount_t read critical section w/o * lockdep and w/o counter stabilization * @s: Pointer to seqcount_t or any of the seqcount_LOCKNAME_t variants @@ -637,6 +645,23 @@ static __always_inline unsigned raw_read_seqcount_latch(const seqcount_latch_t * } /** + * read_seqcount_latch() - pick even/odd latch data copy + * @s: Pointer to seqcount_latch_t + * + * See write_seqcount_latch() for details and a full reader/writer usage + * example. + * + * Return: sequence counter raw value. Use the lowest bit as an index for + * picking which data copy to read. The full counter must then be checked + * with read_seqcount_latch_retry(). + */ +static __always_inline unsigned read_seqcount_latch(const seqcount_latch_t *s) +{ + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); + return raw_read_seqcount_latch(s); +} + +/** * raw_read_seqcount_latch_retry() - end a seqcount_latch_t read section * @s: Pointer to seqcount_latch_t * @start: count, from raw_read_seqcount_latch() @@ -651,8 +676,33 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) } /** + * read_seqcount_latch_retry() - end a seqcount_latch_t read section + * @s: Pointer to seqcount_latch_t + * @start: count, from read_seqcount_latch() + * + * Return: true if a read section retry is required, else false + */ +static __always_inline int +read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) +{ + kcsan_atomic_next(0); + return raw_read_seqcount_latch_retry(s, start); +} + +/** * raw_write_seqcount_latch() - redirect latch readers to even/odd copy * @s: Pointer to seqcount_latch_t + */ +static __always_inline void raw_write_seqcount_latch(seqcount_latch_t *s) +{ + smp_wmb(); /* prior stores before incrementing "sequence" */ + s->seqcount.sequence++; + smp_wmb(); /* increment "sequence" before following stores */ +} + +/** + * write_seqcount_latch_begin() - redirect latch readers to odd copy + * @s: Pointer to seqcount_latch_t * * The latch technique is a multiversion concurrency control method that allows * queries during non-atomic modifications. If you can guarantee queries never @@ -680,17 +730,11 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) * * void latch_modify(struct latch_struct *latch, ...) * { - * smp_wmb(); // Ensure that the last data[1] update is visible - * latch->seq.sequence++; - * smp_wmb(); // Ensure that the seqcount update is visible - * + * write_seqcount_latch_begin(&latch->seq); * modify(latch->data[0], ...); - * - * smp_wmb(); // Ensure that the data[0] update is visible - * latch->seq.sequence++; - * smp_wmb(); // Ensure that the seqcount update is visible - * + * write_seqcount_latch(&latch->seq); * modify(latch->data[1], ...); + * write_seqcount_latch_end(&latch->seq); * } * * The query will have a form like:: @@ -701,13 +745,13 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) * unsigned seq, idx; * * do { - * seq = raw_read_seqcount_latch(&latch->seq); + * seq = read_seqcount_latch(&latch->seq); * * idx = seq & 0x01; * entry = data_query(latch->data[idx], ...); * * // This includes needed smp_rmb() - * } while (raw_read_seqcount_latch_retry(&latch->seq, seq)); + * } while (read_seqcount_latch_retry(&latch->seq, seq)); * * return entry; * } @@ -731,11 +775,31 @@ raw_read_seqcount_latch_retry(const seqcount_latch_t *s, unsigned start) * 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_latch_t *s) +static __always_inline void write_seqcount_latch_begin(seqcount_latch_t *s) { - smp_wmb(); /* prior stores before incrementing "sequence" */ - s->seqcount.sequence++; - smp_wmb(); /* increment "sequence" before following stores */ + kcsan_nestable_atomic_begin(); + raw_write_seqcount_latch(s); +} + +/** + * write_seqcount_latch() - redirect latch readers to even copy + * @s: Pointer to seqcount_latch_t + */ +static __always_inline void write_seqcount_latch(seqcount_latch_t *s) +{ + raw_write_seqcount_latch(s); +} + +/** + * write_seqcount_latch_end() - end a seqcount_latch_t write section + * @s: Pointer to seqcount_latch_t + * + * Marks the end of a seqcount_latch_t writer section, after all copies of the + * latch-protected data have been updated. + */ +static __always_inline void write_seqcount_latch_end(seqcount_latch_t *s) +{ + kcsan_nestable_atomic_end(); } #define __SEQLOCK_UNLOCKED(lockname) \ @@ -769,11 +833,7 @@ static inline void raw_write_seqcount_latch(seqcount_latch_t *s) */ static inline unsigned read_seqbegin(const seqlock_t *sl) { - unsigned ret = read_seqcount_begin(&sl->seqcount); - - kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry() */ - kcsan_flat_atomic_begin(); - return ret; + return read_seqcount_begin(&sl->seqcount); } /** @@ -789,12 +849,6 @@ static inline unsigned read_seqbegin(const seqlock_t *sl) */ static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) { - /* - * Assume not nested: read_seqretry() may be called multiple times when - * completing read critical section. - */ - kcsan_flat_atomic_end(); - return read_seqcount_retry(&sl->seqcount, start); } |