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

[Leon Romanovsky <leon@kernel.org>]

On Thu, Oct 24, 2019 at 03:36:39PM -0300, Jason Gunthorpe wrote:
> On Thu, Oct 24, 2019 at 06:28:35PM +0000, Parav Pandit wrote:
> >
> >
> > > From: Leon Romanovsky <leon@kernel.org>
> > > Sent: Thursday, October 24, 2019 11:13 AM
> > > To: Jason Gunthorpe <jgg@ziepe.ca>
> > > Cc: Doug Ledford <dledford@redhat.com>; Parav Pandit
> > > <parav@mellanox.com>; RDMA mailing list <linux-rdma@vger.kernel.org>
> > > Subject: Re: [PATCH rdma-next] IB/core: Avoid deadlock during netlink message
> > > handling
> > >
> > > On Thu, Oct 24, 2019 at 01:08:10PM -0300, Jason Gunthorpe wrote:
> > > > 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..
>
> > Its better that they are hidden. So that we don't need open code
> > them.
>
> I wouldn't call swapping one function call for another 'open coding'
>
> > Also with srcu, we don't need lock annotations in get_cb_table()
> > which releases and acquires semaphore.
>
> You don't need lock annoations for that.
>
> > Additionally lock nesting makes overall more complex.
>
> SRCU nesting is just as complicated! Don't think SRCU magically hides
> that issue, it is still proposing to nest SRCU read side sections.
>
> > Given that there are only 3 indices, out of which only 2 are outside
> > of the ib_core module and unlikely to be unloaded, I also prefer
> > srcu version.
>
> Why? It isn't faster, it uses more memory, it still has the same
> complex concurrency arrangement..

Jason,

It doesn't worth arguing, both Parav and I prefer SRCU variant, you
prefer rwsem, so go for it, take rwsem, it is not important.

Thanks

>
> Jason