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>
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 1c0cf31..890c7ef 100644
--- a/include/linux/seqlock.h
+++ b/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>
/*
@@ -233,6 +234,11 @@
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 @@
* unsigned seq, idx;
*
* do {
- * seq = latch->seq;
- * smp_rmb();
+ * seq = lockless_dereference(latch->seq);
*
* idx = seq & 0x01;
* entry = data_query(latch->data[idx], ...);
