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SubjectRe: [PATCH 3/8] RDMAVT: Fix synchronization around percpu_ref
On Wed, Mar 14, 2018 at 12:45:10PM -0700, Tejun Heo wrote:
> 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 <>
> Acked-by: Dennis Dalessandro <>
> Cc: Mike Marciniszyn <>
> Cc:
> Cc: Linus Torvalds <>
> drivers/infiniband/sw/rdmavt/mr.c | 10 ++++++----
> 1 file changed, 6 insertions(+), 4 deletions(-)

Applied to rdma for-next


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