linux-kernel - Re: [PATCH 1/3] infiniband: hw: hfi1: verbs.c: Use built-in RCU list checking

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Date:   Tue, 14 Jan 2020 15:41:48 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Dennis Dalessandro <dennis.dalessandro@...el.com>
Cc:     madhuparnabhowmik04@...il.com, mike.marciniszyn@...el.com,
        paulmck@...nel.org, joel@...lfernandes.org, frextrite@...il.com,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        rcu@...r.kernel.org, linux-rdma@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] infiniband: hw: hfi1: verbs.c: Use built-in RCU list
 checking

On Tue, Jan 14, 2020 at 01:24:00PM -0500, Dennis Dalessandro wrote:
> On 1/14/2020 12:00 PM, Dennis Dalessandro wrote:
> > On 1/14/2020 11:57 AM, Jason Gunthorpe wrote:
> > > On Tue, Jan 14, 2020 at 09:53:45PM +0530,
> > > madhuparnabhowmik04@...il.com wrote:
> > > > From: Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
> > > > 
> > > > list_for_each_entry_rcu has built-in RCU and lock checking.
> > > > Pass cond argument to list_for_each_entry_rcu.
> > > > 
> > > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@...il.com>
> > > >   drivers/infiniband/hw/hfi1/verbs.c | 2 +-
> > > >   1 file changed, 1 insertion(+), 1 deletion(-)
> > > > 
> > > > diff --git a/drivers/infiniband/hw/hfi1/verbs.c
> > > > b/drivers/infiniband/hw/hfi1/verbs.c
> > > > index 089e201d7550..22f2d4fd2577 100644
> > > > +++ b/drivers/infiniband/hw/hfi1/verbs.c
> > > > @@ -515,7 +515,7 @@ static inline void hfi1_handle_packet(struct
> > > > hfi1_packet *packet,
> > > >                          opa_get_lid(packet->dlid, 9B));
> > > >           if (!mcast)
> > > >               goto drop;
> > > > -        list_for_each_entry_rcu(p, &mcast->qp_list, list) {
> > > > +        list_for_each_entry_rcu(p, &mcast->qp_list, list,
> > > > lockdep_is_held(&(ibp->rvp.lock))) {
> > > 
> > > Okay, this looks reasonable
> > > 
> > > Mike, Dennis, is this the right lock to test?
> > > 
> > 
> > I'm looking at that right now actually, I don't think this is correct.
> > Wanted to talk to Mike before I send a response though.
> > 
> > -Denny
> 
> That's definitely going to throw a ton of lock dep messages. It's not really
> the right lock either. Instead what we probably need to do is what we do in
> the non-multicast part of the code and take the rcu_read_lock().

Uh.. why is this using the _rcu varient without holding the rcu lock?
That is quite wrong already.

Jason