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author | Nick Piggin <npiggin@kernel.dk> | 2011-01-07 17:49:51 +1100 |
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committer | Nick Piggin <npiggin@kernel.dk> | 2011-01-07 17:50:27 +1100 |
commit | 3c22cd5709e8143444a6d08682a87f4c57902df3 (patch) | |
tree | 6d245491d9103a1d77a3128ed5d1d996d3794300 /include | |
parent | ff0c7d15f9787b7e8c601533c015295cc68329f8 (diff) | |
download | linux-3.10-3c22cd5709e8143444a6d08682a87f4c57902df3.tar.gz linux-3.10-3c22cd5709e8143444a6d08682a87f4c57902df3.tar.bz2 linux-3.10-3c22cd5709e8143444a6d08682a87f4c57902df3.zip |
kernel: optimise seqlock
Add branch annotations for seqlock read fastpath, and introduce
__read_seqcount_begin and __read_seqcount_end functions, that can avoid the
smp_rmb() if used carefully. These will be used by store-free path walking
algorithm performance is critical and seqlocks are in use.
Signed-off-by: Nick Piggin <npiggin@kernel.dk>
Diffstat (limited to 'include')
-rw-r--r-- | include/linux/seqlock.h | 80 |
1 files changed, 73 insertions, 7 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 632205ccc25..e98cd2e5719 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -107,7 +107,7 @@ static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start) { smp_rmb(); - return (sl->sequence != start); + return unlikely(sl->sequence != start); } @@ -125,14 +125,25 @@ typedef struct seqcount { #define SEQCNT_ZERO { 0 } #define seqcount_init(x) do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0) -/* Start of read using pointer to a sequence counter only. */ -static inline unsigned read_seqcount_begin(const seqcount_t *s) +/** + * __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 = s->sequence; - smp_rmb(); if (unlikely(ret & 1)) { cpu_relax(); goto repeat; @@ -140,14 +151,56 @@ repeat: return ret; } -/* - * Test if reader processed invalid data because sequence number has changed. +/** + * 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) +{ + unsigned ret = __read_seqcount_begin(s); + smp_rmb(); + return ret; +} + +/** + * __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 s->sequence != start; + return __read_seqcount_retry(s, start); } @@ -167,6 +220,19 @@ static inline void write_seqcount_end(seqcount_t *s) s->sequence++; } +/** + * write_seqcount_barrier - invalidate in-progress read-side seq operations + * @s: pointer to seqcount_t + * + * After write_seqcount_barrier, no read-side seq operations will complete + * successfully and see data older than this. + */ +static inline void write_seqcount_barrier(seqcount_t *s) +{ + smp_wmb(); + s->sequence+=2; +} + /* * Possible sw/hw IRQ protected versions of the interfaces. */ |