[PATCH 2/2] nvmet: don't allow I/O admission after percpu ns reference is killed

[Nilay Shroff <nilay@linux.ibm.com>]

nvmet_req_find_ns() uses percpu_ref_get() to obtain a reference to
the namespace. However, percpu_ref_get() can acquire a reference even
after the namespace reference has been killed (or marked DEAD).

This is undesirable during namespace disable because nvmet_ns_disable()
kills the namespace reference and then waits for all outstanding
references to drain. Acquiring a new reference after the reference is
killed can therefore extend the namespace drain period.

Replace percpu_ref_get() in nvmet_req_find_ns() with
percpu_ref_tryget_live_rcu(), which only acquires a reference while
the namespace reference is still live. This handles the race where
nvmet_req_find_ns() finds ns is enabled but before it acquires the
reference to ns, its reference is killed in nvmet_ns_disable().
For instnace check this race:

CPU0                              CPU1
nvmet_req_find_ns():              nvmet_ns_disable():
xa_load() -> ns
mark == set

                                  xa_clear_mark()
                                  percpu_ref_kill() // DEAD

percpu_ref_get()                  synchronize_rcu()
        |                         wait_for_completion()
        +-- succeed

Replacing percpu_ref_get() with percpu_ref_tryget_live_rcu() prevents
the I/O request from acquiring a namespace reference once the
reference has been marked DEAD.

Perform the namespace lookup and reference acquisition in
nvmet_req_find_ns() within an RCU read-side critical section.
nvmet_ns_disable() uses synchronize_rcu() before draining and exiting
the namespace reference, ensuring that RCU readers which may be
acquiring the namespace reference have completed before the reference
is exited.

Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
---
 drivers/nvme/target/core.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/drivers/nvme/target/core.c b/drivers/nvme/target/core.c
index 77d113fceced..81a2ae072909 100644
--- a/drivers/nvme/target/core.c
+++ b/drivers/nvme/target/core.c
@@ -446,21 +446,27 @@ u16 nvmet_req_find_ns(struct nvmet_req *req)
 {
 	u32 nsid = le32_to_cpu(req->cmd->common.nsid);
 	struct nvmet_subsys *subsys = nvmet_req_subsys(req);
+	u16 status = NVME_SC_SUCCESS;
 
+	rcu_read_lock();
 	req->ns = xa_load(&subsys->namespaces, nsid);
 	if (unlikely(!req->ns) ||
-	    !xa_get_mark(&subsys->namespaces, nsid, NVMET_NS_ENABLED)) {
+	    !xa_get_mark(&subsys->namespaces, nsid, NVMET_NS_ENABLED) ||
+	    !percpu_ref_tryget_live_rcu(&req->ns->ref)) {
 		req->error_loc = offsetof(struct nvme_common_command, nsid);
-		if (!req->ns) /* ns doesn't exist! */
-			return NVME_SC_INVALID_NS | NVME_STATUS_DNR;
+		if (!req->ns) { /* ns doesn't exist! */
+			status = NVME_SC_INVALID_NS | NVME_STATUS_DNR;
+			goto unlock;
+		}
 
 		/* ns exists but it's disabled */
 		req->ns = NULL;
-		return NVME_SC_INTERNAL_PATH_ERROR;
+		status = NVME_SC_INTERNAL_PATH_ERROR;
 	}
+unlock:
+	rcu_read_unlock();
 
-	percpu_ref_get(&req->ns->ref);
-	return NVME_SC_SUCCESS;
+	return status;
 }
 
 static void nvmet_destroy_namespace(struct percpu_ref *ref)
-- 
2.53.0