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 the fast exit and
adds an explicit synchronize_rcu().
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Cc: Dennis Dalessandro <dennis.dalessandro@xxxxxxxxx>
Cc: Mike Marciniszyn <mike.marciniszyn@xxxxxxxxx>
Cc: linux-rdma@xxxxxxxxxxxxxxx
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
---
Hello, Dennis, Mike.
I don't know RDMA at all and this patch is only compile tested. Can
you please take a careful look?
Thanks.
drivers/infiniband/sw/rdmavt/mr.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)
diff --git a/drivers/infiniband/sw/rdmavt/mr.c b/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 */
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,
--
2.9.5
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