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Commit 7fc26327 authored by Peter Zijlstra's avatar Peter Zijlstra Committed by Rusty Russell
Browse files

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: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 0a04b016

include/linux/seqlock.h

+7 −2
Original line number Diff line number Diff line
@@ -35,6 +35,7 @@ 
#include <linux/spinlock.h>

#include <linux/preempt.h>

#include <linux/lockdep.h>

#include <linux/compiler.h>

#include <asm/processor.h>



/*
@@ -233,6 +234,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s) 
	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
@@ -284,8 +290,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s) 
 *	unsigned seq, idx;

 *

 *	do {

 *		seq = latch->seq;

 *		smp_rmb();

 *		seq = lockless_dereference(latch->seq);

 *

 *		idx = seq & 0x01;

 *		entry = data_query(latch->data[idx], ...);

kernel/time/timekeeping.c

+1 −1
Original line number Diff line number Diff line
@@ -393,7 +393,7 @@ static __always_inline u64 __ktime_get_fast_ns(struct tk_fast *tkf) 
	u64 now;



	do {

		seq = raw_read_seqcount(&tkf->seq);

		seq = raw_read_seqcount_latch(&tkf->seq);

		tkr = tkf->base + (seq & 0x01);

		now = ktime_to_ns(tkr->base) + timekeeping_get_ns(tkr);

	} while (read_seqcount_retry(&tkf->seq, seq));