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

* 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 the fast exit and
adds an explicit synchronize_rcu().

Signed-off-by: Tejun Heo <t...@kernel.org>
Cc: Dennis Dalessandro <dennis.dalessan...@intel.com>
Cc: Mike Marciniszyn <mike.marcinis...@intel.com>
Cc: linux-r...@vger.kernel.org
Cc: Linus Torvalds <torva...@linux-foundation.org>
Hello, Dennis, Mike.

I don't know RDMA at all and this patch is only compile tested.  Can
you please take a careful look?


 drivers/infiniband/sw/rdmavt/mr.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/infiniband/sw/rdmavt/mr.c 
index 1b2e536..cc429b5 100644
--- 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 */
+               /* @mr was indexed on rcu protected @lkey_table */
+               synchronize_rcu();
+       }
        timeout = wait_for_completion_timeout(&mr->comp, 5 * HZ);
        if (!timeout) {

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