[PATCH] dlm: wait for outstanding SRCU callbacks to complete in exit paths

[Zqiang]

The dlm_lowcomms_exit() and dlm_midcomms_exit() iterate over the
srcu protected connection and node hash tables and hand each
element to call_srcu() for deferred freeing (connection_release()
and midcomms_node_release()). call_srcu() is asynchronous: the
callbacks are invoked only after an SRCU grace period, which may
happen after the exit function has already returned.

These exit functions are reached from exit_dlm() on module unload.
Once they return, module teardown continues and the module text
may be unloaded while call_srcu() callbacks are still pending. When
such a callback finally runs, it executes freed module code and
touches the static SRCU domains that are being torn down, resulting
in a use-after-free.

Add an srcu_barrier() after the call_srcu() loop in each exit function
to wait for all outstanding callbacks of the respective SRCU domain to
complete before returning. In dlm_midcomms_exit() the barrier is issued
before dlm_lowcomms_exit() so that node callbacks are drained prior to
tearing down the lower layer.

Signed-off-by: Zqiang <qiang.zhang@xxxxxxxxx>
---
 fs/dlm/lowcomms.c | 1 +
 fs/dlm/midcomms.c | 1 +
 2 files changed, 2 insertions(+)

diff --git a/fs/dlm/lowcomms.c b/fs/dlm/lowcomms.c
index 2aff1c7c17de..ea8353c4638d 100644
--- a/fs/dlm/lowcomms.c
+++ b/fs/dlm/lowcomms.c
@@ -1984,4 +1984,5 @@ void dlm_lowcomms_exit(void)
 		}
 	}
 	srcu_read_unlock(&connections_srcu, idx);
+	srcu_barrier(&connections_srcu);
 }
diff --git a/fs/dlm/midcomms.c b/fs/dlm/midcomms.c
index 8964164600d2..045431524494 100644
--- a/fs/dlm/midcomms.c
+++ b/fs/dlm/midcomms.c
@@ -1178,6 +1178,7 @@ void dlm_midcomms_exit(void)
 		}
 	}
 	srcu_read_unlock(&nodes_srcu, idx);
+	srcu_barrier(&nodes_srcu);
 
 	dlm_lowcomms_exit();
 }
-- 
2.17.1