[PATCH] percpu_ida: Handle out-of-tags gracefully

[Bart Van Assche <bvanassche@acm.org>]

Avoid that percpu_ida_alloc() hangs or crashes if there are still
tags are available. 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 ]---

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Shaohua Li <shli@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Alexander Gordeev <agordeev@redhat.com>
Cc: Mike Christie <michaelc@cs.wisc.edu>
---
 lib/percpu_ida.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/lib/percpu_ida.c b/lib/percpu_ida.c
index 93d145e..170d27c 100644
--- a/lib/percpu_ida.c
+++ b/lib/percpu_ida.c
@@ -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;
@@ -189,6 +189,9 @@ int percpu_ida_alloc(struct percpu_ida *pool, int state)
 		spin_unlock(&pool->lock);
 		local_irq_restore(flags);
 
+		if (tags->nr_free)
+			wake_up(&pool->wait);
+
 		if (tag >= 0 || state == TASK_RUNNING)
 			break;
 
-- 
1.8.4.5