Re: [PATCH 3/8] RDMAVT: Fix synchronization around percpu_ref

[Jason Gunthorpe]

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 <tj@xxxxxxxxxx>
> Acked-by: Dennis Dalessandro <dennis.dalessandro@xxxxxxxxx>
> Cc: Mike Marciniszyn <mike.marciniszyn@xxxxxxxxx>
> Cc: linux-rdma@xxxxxxxxxxxxxxx
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
>  drivers/infiniband/sw/rdmavt/mr.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)

Applied to rdma for-next

Thanks,
Jason