blk-mq: Don't reserve a tag for flush request

Reserving a tag (request) for flush to avoid dead lock is a overkill. A
tag is valuable resource. We can track the number of flush requests and
disallow having too many pending flush requests allocated. With this
patch, blk_mq_alloc_request_pinned() could do a busy nop (but not a dead
loop) if too many pending requests are allocated and new flush request
is allocated. But this should not be a problem, too many pending flush
requests are very rare case.

I verified this can fix the deadlock caused by too many pending flush
requests.

Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 3 insertions, 0 deletions

diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 161b23105b1e..1e8f16f65af4 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -36,12 +36,15 @@ struct blk_mq_hw_ctx {
         struct list_head        page_list;
         struct blk_mq_tags      *tags;
 
+        atomic_t                pending_flush;
+
         unsigned long           queued;
         unsigned long           run;
 #define BLK_MQ_MAX_DISPATCH_ORDER       10
         unsigned long           dispatched[BLK_MQ_MAX_DISPATCH_ORDER];
 
         unsigned int            queue_depth;
+        unsigned int            reserved_tags;
         unsigned int            numa_node;
         unsigned int            cmd_size;       /* per-request extra data */