[PATCH] x86/tsc: use real seqcount_latch in cyc2ns_read_begin()

[Eric Dumazet <edumazet@google.com>]

While looking at native_sched_clock() disassembly I had
the surprise to see the compiler (gcc 7.3 here) had
optimized out the loop, meaning the code is broken.

Using the documented and approved API not only fixes the bug,
it also makes the code more readable.

Replacing five this_cpu_read() by one this_cpu_ptr() makes
the generated code smaller.

Fixes: 59eaef78bfea ("x86/tsc: Remodel cyc2ns to use seqcount_latch()")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
---
 arch/x86/kernel/tsc.c | 15 ++++++---------
 1 file changed, 6 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index b52bd2b6cdb443ba0c89d78aaa52b02b82a10b6e..b039ae0f358a4f78cc830c069ad90e4fb108489e 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -60,19 +60,16 @@ static DEFINE_PER_CPU_ALIGNED(struct cyc2ns, cyc2ns);
 
 void cyc2ns_read_begin(struct cyc2ns_data *data)
 {
-	int seq, idx;
+	const struct cyc2ns *c2n;
+	int seq;
 
 	preempt_disable_notrace();
 
+	c2n = this_cpu_ptr(&cyc2ns);
 	do {
-		seq = this_cpu_read(cyc2ns.seq.sequence);
-		idx = seq & 1;
-
-		data->cyc2ns_offset = this_cpu_read(cyc2ns.data[idx].cyc2ns_offset);
-		data->cyc2ns_mul    = this_cpu_read(cyc2ns.data[idx].cyc2ns_mul);
-		data->cyc2ns_shift  = this_cpu_read(cyc2ns.data[idx].cyc2ns_shift);
-
-	} while (unlikely(seq != this_cpu_read(cyc2ns.seq.sequence)));
+		seq = raw_read_seqcount_latch(&c2n->seq);
+		*data = c2n->data[seq & 1];
+	} while (read_seqcount_retry(&c2n->seq, seq));
 }
 
 void cyc2ns_read_end(void)
-- 
2.19.0.605.g01d371f741-goog