percpu_ida: Handle out-of-tags gracefully

Wait until a tag becomes available instead of giving up when running out of tags temporarily. This patch fixes the following kernel bug:

------------[ cut here ]------------
kernel BUG at lib/percpu_ida.c:81!
invalid opcode: 0000 [#1] SMP
RIP: 0010:[<ffffffff8120f00e>]  [<ffffffff8120f00e>] percpu_ida_alloc+0x33e/0x370
Call Trace:
 [<ffffffff811ef95f>] blk_mq_get_tag+0x2f/0x50
 [<ffffffff811ed79c>] blk_mq_alloc_rq.isra.17+0x1c/0x90
 [<ffffffff811eeb9b>] blk_mq_alloc_request_pinned+0x9b/0x110
 [<ffffffff811ef4c6>] blk_mq_make_request+0x426/0x480
 [<ffffffff811e28f0>] generic_make_request+0xc0/0x110
 [<ffffffff811e29ab>] submit_bio+0x6b/0x140
 [<ffffffff8117aabb>] _submit_bh+0x13b/0x220
 [<ffffffff8117d70f>] block_read_full_page+0x1ff/0x300
 [<ffffffff81181128>] blkdev_readpage+0x18/0x20
 [<ffffffff811067b7>] __do_page_cache_readahead+0x277/0x280
 [<ffffffff81106d1d>] force_page_cache_readahead+0x8d/0xc0
 [<ffffffff81106d9b>] page_cache_sync_readahead+0x4b/0x50
 [<ffffffff810fdf05>] generic_file_aio_read+0x4c5/0x700
 [<ffffffff8118147b>] blkdev_aio_read+0x4b/0x70
 [<ffffffff8114a28a>] do_sync_read+0x5a/0x90
 [<ffffffff8114a8cb>] vfs_read+0x9b/0x160
 [<ffffffff8114b389>] SyS_read+0x49/0xa0
 [<ffffffff81416049>] tracesys+0xd0/0xd5
---[ end trace cdd1a8a7968266cf ]---

---
 lib/percpu_ida.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 93d145e..56b5067 100644
@@ -73,7 +73,7 @@ static inline void steal_tags(struct percpu_ida *pool,
 		if (cpu >= nr_cpu_ids) {
 			cpu = cpumask_first(&pool->cpus_have_tags);
 			if (cpu >= nr_cpu_ids)
-				BUG();
+				break;
 		}
 
 		pool->cpu_last_stolen = cpu;

1.8.4.5