blk-mq: fix for flush deadlock
The flush state machine takes in a struct request, which then is
submitted multiple times to the underling driver. The old block code
requeses the same request for each of those, so it does not have an
issue with tapping into the request pool. The new one on the other hand
allocates a new request for each of the actualy steps of the flush
sequence. If have already allocated all of the tags for IO, we will
fail allocating the flush request.
Set aside a reserved request just for flushes.
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ac804c6..2dc8de8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -210,14 +210,15 @@
return rq;
}
-struct request *blk_mq_alloc_request(struct request_queue *q, int rw, gfp_t gfp)
+struct request *blk_mq_alloc_request(struct request_queue *q, int rw,
+ gfp_t gfp, bool reserved)
{
struct request *rq;
if (blk_mq_queue_enter(q))
return NULL;
- rq = blk_mq_alloc_request_pinned(q, rw, gfp, false);
+ rq = blk_mq_alloc_request_pinned(q, rw, gfp, reserved);
blk_mq_put_ctx(rq->mq_ctx);
return rq;
}
@@ -1327,6 +1328,15 @@
reg->queue_depth = BLK_MQ_MAX_DEPTH;
}
+ /*
+ * Set aside a tag for flush requests. It will only be used while
+ * another flush request is in progress but outside the driver.
+ *
+ * TODO: only allocate if flushes are supported
+ */
+ reg->queue_depth++;
+ reg->reserved_tags++;
+
if (reg->queue_depth < (reg->reserved_tags + BLK_MQ_TAG_MIN))
return ERR_PTR(-EINVAL);