[PATCH v6 1/6] nbd: don't handle response without a corresponding request message

Yu Kuai
2021-09-15T08:05:54Z

While handling a response message from server, nbd_read_stat() will
try to get request by tag, and then complete the request. However,
this is problematic if nbd haven't sent a corresponding request
message:

t1 t2
submit_bio
nbd_queue_rq
blk_mq_start_request
recv_work
nbd_read_stat
blk_mq_tag_to_rq
blk_mq_complete_request
nbd_send_cmd

Thus add a new cmd flag 'NBD_CMD_INFLIGHT', it will be set in
nbd_send_cmd() and checked in nbd_read_stat().

Noted that this patch can't fix that blk_mq_tag_to_rq() might
return a freed request, and this will be fixed in following
patches.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
---
drivers/block/nbd.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 5170a630778d..23ded569e8d3 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -126,6 +126,12 @@ struct nbd_device {
};

#define NBD_CMD_REQUEUED 1
+/*
+ * This flag will be set if nbd_queue_rq() succeed, and will be checked and
+ * cleared in completion. Both setting and clearing of the flag are protected
+ * by cmd->lock.
+ */
+#define NBD_CMD_INFLIGHT 2

struct nbd_cmd {
struct nbd_device *nbd;
@@ -400,6 +406,7 @@ static enum blk_eh_timer_return nbd_xmit_timeout(struct request *req,
if (!mutex_trylock(&cmd->lock))
return BLK_EH_RESET_TIMER;

+ __clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
if (!refcount_inc_not_zero(&nbd->config_refs)) {
cmd->status = BLK_STS_TIMEOUT;
mutex_unlock(&cmd->lock);
@@ -729,6 +736,12 @@ static struct nbd_cmd *nbd_read_stat(struct nbd_device *nbd, int index)
cmd = blk_mq_rq_to_pdu(req);

mutex_lock(&cmd->lock);
+ if (!__test_and_clear_bit(NBD_CMD_INFLIGHT, &cmd->flags)) {
+ dev_err(disk_to_dev(nbd->disk), "Suspicious reply %d (status %u flags %lu)",
+ tag, cmd->status, cmd->flags);
+ ret = -ENOENT;
+ goto out;
+ }
if (cmd->cmd_cookie != nbd_handle_to_cookie(handle)) {
dev_err(disk_to_dev(nbd->disk), "Double reply on req %p, cmd_cookie %u, handle cookie %u\n",
req, cmd->cmd_cookie, nbd_handle_to_cookie(handle));
@@ -828,6 +841,7 @@ static bool nbd_clear_req(struct request *req, void *data, bool reserved)
return true;

mutex_lock(&cmd->lock);
+ __clear_bit(NBD_CMD_INFLIGHT, &cmd->flags);
cmd->status = BLK_STS_IOERR;
mutex_unlock(&cmd->lock);

@@ -964,7 +978,13 @@ static int nbd_handle_cmd(struct nbd_cmd *cmd, int index)
* returns EAGAIN can be retried on a different socket.
*/
ret = nbd_send_cmd(nbd, cmd, index);
- if (ret == -EAGAIN) {
+ /*
+ * Access to this flag is protected by cmd->lock, thus it's safe to set
+ * the flag after nbd_send_cmd() succeed to send request to server.
+ */
+ if (!ret)
+ __set_bit(NBD_CMD_INFLIGHT, &cmd->flags);
+ else if (ret == -EAGAIN) {
dev_err_ratelimited(disk_to_dev(nbd->disk),
"Request send failed, requeueing\n");
nbd_mark_nsock_dead(nbd, nsock, 1);
--
2.31.1