x86/tsc: Use seqcount_latch_t

Latch sequence counters have unique read and write APIs, and thus
seqcount_latch_t was recently introduced at seqlock.h.

Use that new data type instead of plain seqcount_t. This adds the
necessary type-safety and ensures that only latching-safe seqcount APIs
are to be used.

Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
[peterz: unwreck cyc2ns_read_begin()]
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200827114044.11173-7-a.darwish@linutronix.de

Author: a-darwish

arch/x86/kernel/tsc.c

@@ -54,7 +54,7 @@ struct clocksource *art_related_clocksource;
 
 struct cyc2ns {
 	struct cyc2ns_data data[2];	/*  0 + 2*16 = 32 */
-	seqcount_t	   seq;		/* 32 + 4    = 36 */
+	seqcount_latch_t   seq;		/* 32 + 4    = 36 */
 
 }; /* fits one cacheline */
 
@@ -73,14 +73,14 @@ __always_inline void cyc2ns_read_begin(struct cyc2ns_data *data)
 	preempt_disable_notrace();
 
 	do {
-		seq = this_cpu_read(cyc2ns.seq.sequence);
+		seq = this_cpu_read(cyc2ns.seq.seqcount.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)));
+	} while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
 }
 
 __always_inline void cyc2ns_read_end(void)
@@ -186,7 +186,7 @@ static void __init cyc2ns_init_boot_cpu(void)
 {
 	struct cyc2ns *c2n = this_cpu_ptr(&cyc2ns);
 
-	seqcount_init(&c2n->seq);
+	seqcount_latch_init(&c2n->seq);
 	__set_cyc2ns_scale(tsc_khz, smp_processor_id(), rdtsc());
 }
 
@@ -203,7 +203,7 @@ static void __init cyc2ns_init_secondary_cpus(void)
 
 	for_each_possible_cpu(cpu) {
 		if (cpu != this_cpu) {
-			seqcount_init(&c2n->seq);
+			seqcount_latch_init(&c2n->seq);
 			c2n = per_cpu_ptr(&cyc2ns, cpu);
 			c2n->data[0] = data[0];
 			c2n->data[1] = data[1];