diff options
author | 2016-04-11 10:41:07 +0300 | |
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committer | 2016-04-13 08:17:18 +0300 | |
commit | e09b41010ba33a20a87472ee821fa407a5b8da36 (patch) | |
tree | d10dc367189862e7ca5c592f033dc3726e1df4e3 /kernel/include/linux/seqlock.h | |
parent | f93b97fd65072de626c074dbe099a1fff05ce060 (diff) |
These changes are the raw update to linux-4.4.6-rt14. Kernel sources
are taken from kernel.org, and rt patch from the rt wiki download page.
During the rebasing, the following patch collided:
Force tick interrupt and get rid of softirq magic(I70131fb85).
Collisions have been removed because its logic was found on the
source already.
Change-Id: I7f57a4081d9deaa0d9ccfc41a6c8daccdee3b769
Signed-off-by: José Pekkarinen <jose.pekkarinen@nokia.com>
Diffstat (limited to 'kernel/include/linux/seqlock.h')
-rw-r--r-- | kernel/include/linux/seqlock.h | 128 |
1 files changed, 124 insertions, 4 deletions
diff --git a/kernel/include/linux/seqlock.h b/kernel/include/linux/seqlock.h index eaade3619..b14f4d236 100644 --- a/kernel/include/linux/seqlock.h +++ b/kernel/include/linux/seqlock.h @@ -35,6 +35,7 @@ #include <linux/spinlock.h> #include <linux/preempt.h> #include <linux/lockdep.h> +#include <linux/compiler.h> #include <asm/processor.h> /* @@ -243,9 +244,128 @@ static inline void raw_write_seqcount_end(seqcount_t *s) preempt_enable_rt(); } -/* +/** + * 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. + * + * 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) + * { + * Y = true; + * + * raw_write_seqcount_barrier(seq); + * + * 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) +{ + return lockless_dereference(s->sequence); +} + +/** * 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 = lockless_dereference(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) { @@ -276,13 +396,13 @@ static inline void write_seqcount_end(seqcount_t *s) } /** - * write_seqcount_barrier - invalidate in-progress read-side seq operations + * write_seqcount_invalidate - invalidate in-progress read-side seq operations * @s: pointer to seqcount_t * - * After write_seqcount_barrier, no read-side seq operations will complete + * After write_seqcount_invalidate, no read-side seq operations will complete * successfully and see data older than this. */ -static inline void write_seqcount_barrier(seqcount_t *s) +static inline void write_seqcount_invalidate(seqcount_t *s) { smp_wmb(); s->sequence+=2; |