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seqlock: Introduce raw_read_seqcount_latch()
Because with latches there is a strict data dependency on the seq load we can avoid the rmb in favour of a read_barrier_depends. Suggested-by: Ingo Molnar <mingo@kernel.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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@ -35,6 +35,7 @@
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#include <linux/spinlock.h>
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#include <linux/preempt.h>
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#include <linux/lockdep.h>
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#include <linux/compiler.h>
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#include <asm/processor.h>
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/*
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@ -233,6 +234,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
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s->sequence++;
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}
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static inline int raw_read_seqcount_latch(seqcount_t *s)
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{
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return lockless_dereference(s->sequence);
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}
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/**
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* raw_write_seqcount_latch - redirect readers to even/odd copy
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* @s: pointer to seqcount_t
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@ -284,8 +290,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
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* unsigned seq, idx;
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*
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* do {
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* seq = latch->seq;
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* smp_rmb();
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* seq = lockless_dereference(latch->seq);
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*
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* idx = seq & 0x01;
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* entry = data_query(latch->data[idx], ...);
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@ -393,7 +393,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf)
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u64 now;
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do {
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seq = raw_read_seqcount(&tkf->seq);
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seq = raw_read_seqcount_latch(&tkf->seq);
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tkr = tkf->base + (seq & 0x01);
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now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);
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} while (read_seqcount_retry(&tkf->seq, seq));
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