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>
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 161b231..1e8f16f 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -36,12 +36,15 @@
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 */