rvt_mregion uses percpu_ref for reference counting and RCU to protect
accesses from lkey_table. When a rvt_mregion needs to be freed, it
first gets unregistered from lkey_table and then rvt_check_refs() is
called to wait for in-flight usages before the rvt_mregion is freed.
rvt_check_refs() seems to have a couple issues.
* It has a fast exit path which tests percpu_ref_is_zero(). However,
a percpu_ref reading zero doesn't mean that the object can be
released. In fact, the ->release() callback might not even have
started executing yet. Proceeding with freeing can lead to
use-after-free.
* lkey_table is RCU protected but there is no RCU grace period in the
free path. percpu_ref uses RCU internally but it's sched-RCU whose
grace periods are different from regular RCU. Also, it generally
isn't a good idea to depend on internal behaviors like this.
To address the above issues, this patch removes the fast exit and adds
an explicit synchronize_rcu().
Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Dennis Dalessandro <dennis.dalessandro@intel.com> Cc: Mike Marciniszyn <mike.marciniszyn@intel.com> Cc: linux-rdma@vger.kernel.org Cc: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>