flex_proportions: make fprop_new_period() hardirq safe

Bernd has reported a lockdep splat from flexible proportions code that is
essentially complaining about the following race:

<timer fires>
run_timer_softirq - we are in softirq context
call_timer_fn
writeout_period
fprop_new_period
write_seqcount_begin(&p->sequence);

<hardirq is raised>
...
blk_mq_end_request()
blk_update_request()
ext4_end_bio()
folio_end_writeback()
__wb_writeout_add()
__fprop_add_percpu_max()
if (unlikely(max_frac < FPROP_FRAC_BASE)) {
fprop_fraction_percpu()
seq = read_seqcount_begin(&p->sequence);
- sees odd sequence so loops indefinitely

Note that a deadlock like this is only possible if the bdi has configured
maximum fraction of writeout throughput which is very rare in general but
frequent for example for FUSE bdis. To fix this problem we have to make
sure write section of the sequence counter is irqsafe.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: a91befde3503 ("lib/flex_proportions.c: remove local_irq_ops in fprop_new_period()")
Signed-off-by: Jan Kara <[e-mail]>
Reported-by: Bernd Schubert <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Reviewed-by: Matthew Wilcox (Oracle) <[e-mail]>
Cc: Joanne Koong <[e-mail]>
Cc: Miklos Szeredi <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

lib/flex_proportions.c

···
 bool fprop_new_period(struct fprop_global *p, int periods)
 {
 	s64 events = percpu_counter_sum(&p->events);
+	unsigned long flags;
 
 	/*
 	 * Don't do anything if there are no events.
 	 */
 	if (events <= 1)
 		return false;
-	preempt_disable_nested();
+	local_irq_save(flags);
 	write_seqcount_begin(&p->sequence);
 	if (periods < 64)
 		events -= events >> periods;
···
 	percpu_counter_add(&p->events, -events);
 	p->period += periods;
 	write_seqcount_end(&p->sequence);
-	preempt_enable_nested();
+	local_irq_restore(flags);
 
 	return true;
 }