Re: [PATCH rdma-next] IB/core: Avoid deadlock during netlink message handling

[Leon Romanovsky <leon@kernel.org>]

On Thu, Oct 24, 2019 at 10:50:17AM -0300, Jason Gunthorpe wrote:
> On Thu, Oct 24, 2019 at 04:26:07PM +0300, Leon Romanovsky wrote:
> > On Thu, Oct 24, 2019 at 10:17:43AM -0300, Jason Gunthorpe wrote:
> > > On Tue, Oct 15, 2019 at 11:07:33AM +0300, Leon Romanovsky wrote:
> > >
> > > > diff --git a/drivers/infiniband/core/netlink.c b/drivers/infiniband/core/netlink.c
> > > > index 81dbd5f41bed..a3507b8be569 100644
> > > > +++ b/drivers/infiniband/core/netlink.c
> > > > @@ -42,9 +42,12 @@
> > > >  #include <linux/module.h>
> > > >  #include "core_priv.h"
> > > >
> > > > -static DEFINE_MUTEX(rdma_nl_mutex);
> > > >  static struct {
> > > > -	const struct rdma_nl_cbs   *cb_table;
> > > > +	const struct rdma_nl_cbs __rcu *cb_table;
> > > > +	/* Synchronizes between ongoing netlink commands and netlink client
> > > > +	 * unregistration.
> > > > +	 */
> > > > +	struct srcu_struct unreg_srcu;
> > >
> > > A srcu in every index is serious overkill for this. Lets just us a
> > > rwsem:
> >
> > I liked previous variant more than rwsem, but it is Parav's patch.
>
> Why? srcu is a huge data structure and slow on unregister

The unregister time is not so important for those IB/core modules.
I liked SRCU because it doesn't have *_ONCE() macros and smb_* calls.

Maybe wrong here, but the extra advantage of SRCU is that we are already
using that mechanism in uverbs and my assumption that SRCU will greatly
enjoy shared grace period.

Thanks

>
> Jason