Re: [PATCH v2 2/2] nbd: convert to use blk_mq_get_rq_by_tag()
From: Ming Lei <hidden>
Date: 2021-08-09 06:28:40
Also in:
lkml
On Mon, Aug 09, 2021 at 11:09:27AM +0800, Yu Kuai wrote:
quoted hunk ↗ jump to hunk
blk_mq_tag_to_rq() might return freed request, use blk_mq_get_rq_by_tag() instead. Signed-off-by: Yu Kuai <redacted> --- drivers/block/nbd.c | 11 ++++++----- 1 file changed, 6 insertions(+), 5 deletions(-)diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c index c38317979f74..9e56975a8eee 100644 --- a/drivers/block/nbd.c +++ b/drivers/block/nbd.c@@ -713,11 +713,10 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index) tag = nbd_handle_to_tag(handle); hwq = blk_mq_unique_tag_to_hwq(tag); if (hwq < nbd->tag_set.nr_hw_queues) - req = blk_mq_tag_to_rq(nbd->tag_set.tags[hwq], - blk_mq_unique_tag_to_tag(tag)); - if (!req || !blk_mq_request_started(req)) { - dev_err(disk_to_dev(nbd->disk), "Unexpected reply (%d) %p\n", - tag, req); + req = blk_mq_get_rq_by_tag(nbd->tag_set.tags[hwq], + blk_mq_unique_tag_to_tag(tag)); + if (!req) { + dev_err(disk_to_dev(nbd->disk), "Unexpected reply %d\n", tag); return ERR_PTR(-ENOENT); } trace_nbd_header_received(req, handle);@@ -779,6 +778,8 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index) } out: trace_nbd_payload_received(req, handle); + if (req) + blk_mq_put_rq_ref(req); mutex_unlock(&cmd->lock); return ret ? ERR_PTR(ret) : cmd;
After blk_mq_put_rq_ref() returns, this request may have been freed, so the returned 'cmd' may have been freed too. As I replied in your another thread, it is driver's responsibility to cover race between normal completion and timeout/error handling, that means the caller of blk_mq_tag_to_rq need to make sure that the request represented by the passed 'tag' can't be freed. I'd suggest to understand why nbd_read_stat()/blk_mq_tag_to_rq() may return one freed request first, who frees the request and how when calling blk_mq_tag_to_rq() before figuring out solutions. Thanks, Ming