Re: [PATCH 3/7] RDMAVT: Fix synchronization around percpu_ref

[Dennis Dalessandro]

On 3/6/2018 12:33 PM, 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 the fast exit and

Typo above too many "the".

> 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?

Looks good to me, passes my basic sanity tests. Thanks!

Acked-by: Dennis Dalessandro <dennis.dalessandro@xxxxxxxxx>