iocost: Use sequence counter with associated spinlock

A sequence counter write side critical section must be protected by some
form of locking to serialize writers. A plain seqcount_t does not
contain the information of which lock must be held when entering a write
side critical section.

Use the new seqcount_spinlock_t data type, which allows to associate a
spinlock with the sequence counter. This enables lockdep to verify that
the spinlock used for writer serialization is held when the write side
critical section is entered.

If lockdep is disabled this lock association is compiled out and has
neither storage size nor runtime overhead.
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Link: https://lkml.kernel.org/r/20200720155530.1173732-21-a.darwish@linutronix.de

block/blk-iocost.c

@@ -406,7 +406,7 @@ struct ioc {
 	enum ioc_running		running;
 	atomic64_t			vtime_rate;
 
-	seqcount_t			period_seqcount;
+	seqcount_spinlock_t		period_seqcount;
 	u32				period_at;	/* wallclock starttime */
 	u64				period_at_vtime; /* vtime starttime */
 
@@ -873,7 +873,6 @@ static void ioc_now(struct ioc *ioc, struct ioc_now *now)
 
 static void ioc_start_period(struct ioc *ioc, struct ioc_now *now)
 {
-	lockdep_assert_held(&ioc->lock);
 	WARN_ON_ONCE(ioc->running != IOC_RUNNING);
 
 	write_seqcount_begin(&ioc->period_seqcount);
@@ -2001,7 +2000,7 @@ static int blk_iocost_init(struct request_queue *q)
 
 	ioc->running = IOC_IDLE;
 	atomic64_set(&ioc->vtime_rate, VTIME_PER_USEC);
-	seqcount_init(&ioc->period_seqcount);
+	seqcount_spinlock_init(&ioc->period_seqcount, &ioc->lock);
 	ioc->period_at = ktime_to_us(ktime_get());
 	atomic64_set(&ioc->cur_period, 0);
 	atomic_set(&ioc->hweight_gen, 0);