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

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Oct 24, 2019 at 07:02:52PM +0300, Leon Romanovsky wrote:
> 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.

It does, they are just hidden under other macros..

> 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.

Hm, I'm not sure it works like that, the grace periods would be consecutive

Jason