RDMAVT: Fix synchronization around percpu_ref

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>

diff --git a/drivers/infiniband/sw/rdmavt/mr.c b/drivers/infiniband/sw/rdmavt/mr.c
index 1b2e5362a3ffeaeb3813707ed37d2ddb386c2f0d..cc429b567d0a4d1a10ee97a61f71b9f59c0d4561 100644 (file)
--- a/drivers/infiniband/sw/rdmavt/mr.c
+++ b/drivers/infiniband/sw/rdmavt/mr.c
@@ -489,11 +489,13 @@ static int rvt_check_refs(struct rvt_mregion *mr, const char *t)
        unsigned long timeout;
        struct rvt_dev_info *rdi = ib_to_rvt(mr->pd->device);
 
-       if (percpu_ref_is_zero(&mr->refcount))
-               return 0;
-       /* avoid dma mr */
-       if (mr->lkey)
+       if (mr->lkey) {
+               /* avoid dma mr */
                rvt_dereg_clean_qps(mr);
+               /* @mr was indexed on rcu protected @lkey_table */
+               synchronize_rcu();
+       }
+
        timeout = wait_for_completion_timeout(&mr->comp, 5 * HZ);
        if (!timeout) {
                rvt_pr_err(rdi,