Re: [PATCH] net: qrtr: ns: Fix the incorrect usage of rcu_read_lock()

[Manivannan Sadhasivam]

Hi Doug,

On Fri, Oct 02, 2020 at 08:28:51AM -0700, Doug Anderson wrote:
> Hi,
> 
> On Fri, Oct 2, 2020 at 7:15 AM Manivannan Sadhasivam
> <manivannan.sadhasivam@xxxxxxxxxx> wrote:
> >
> > The rcu_read_lock() is not supposed to lock the kernel_sendmsg() API
> > since it has the lock_sock() in qrtr_sendmsg() which will sleep. Hence,
> > fix it by excluding the locking for kernel_sendmsg().
> >
> > Fixes: a7809ff90ce6 ("net: qrtr: ns: Protect radix_tree_deref_slot() using rcu read locks")
> > Reported-by: Doug Anderson <dianders@xxxxxxxxxxxx>
> > Tested-by: Alex Elder <elder@xxxxxxxxxx>
> > Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@xxxxxxxxxx>
> > ---
> >  net/qrtr/ns.c | 20 ++++++++++++++------
> >  1 file changed, 14 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/qrtr/ns.c b/net/qrtr/ns.c
> > index 934999b56d60..0515433de922 100644
> > --- a/net/qrtr/ns.c
> > +++ b/net/qrtr/ns.c
> > @@ -203,15 +203,17 @@ static int announce_servers(struct sockaddr_qrtr *sq)
> >         /* Announce the list of servers registered in this node */
> >         radix_tree_for_each_slot(slot, &node->servers, &iter, 0) {
> >                 srv = radix_tree_deref_slot(slot);
> > +               rcu_read_unlock();
> 
> My RCU-fu is mediocre at best and my radix-tree knowledge is
> non-existent.  However:
> 
> => Reading through radix_tree_deref_slot() it says that if you are
> only holding the read lock that you need to be calling
> radix_tree_deref_retry().  Why don't I see that here?
> 

Well, I drew inspiration from peer drivers and didn't look into the API
documentation properly, my bad :(

> => Without any real knowledge, it seems super sketchy to drop the lock
> while iterating over the tree.  Somehow that feels unsafe.  Hrm, there
> seems to be a function radix_tree_iter_resume() that might be exactly
> what you want, but I'm not totally sure.  The only user I can see
> in-tree (other than radix tree regression testing) is btrfs-tests.c
> but it's using it together with radix_tree_deref_slot_protected().
> 
> In any case, my totally untested and totally knowedge-free proposal
> would look something like this:
> 
>   rcu_read_lock();
>   /* Announce the list of servers registered in this node */
>   radix_tree_for_each_slot(slot, &node->servers, &iter, 0) {
>     srv = radix_tree_deref_slot(slot);
>     if (!srv)
>       continue;
>     if (radix_tree_deref_retry(srv)) {
>       slot = radix_tree_iter_retry(&iter);
>       continue;
>     }
>     slot = radix_tree_iter_resume(slot, &iter);
>     rcu_read_unlock();
> 
>     ret = service_announce_new(sq, srv);
>     if (ret < 0) {
>       pr_err("failed to announce new service\n");
>       return ret;
>     }
> 
>     rcu_read_lock();
>   }
> 
>   rcu_read_unlock();
> 
> What a beast!  Given that this doesn't seem to be what anyone else in
> the kernel is doing exactly, it makes me suspect that there's a more
> fundamental design issue here, though...
> 

That's how it is supposed to be. So I'm going to roll out next revision with
your suggestion for the rest of the deref_slot() calls also.

Thanks for your time looking into this.

Regards,
Mani

> -Doug