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

From: Dennis Dalessandro <dennis.dalessandro@intel.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: mike.marciniszyn@intel.com, paulmck@kernel.org,
	linux-rdma@vger.kernel.org, linux-kernel@vger.kernel.org,
	rcu@vger.kernel.org, joel@joelfernandes.org,
	linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [Linux-kernel-mentees] [PATCH 1/3] infiniband: hw: hfi1: verbs.c: Use built-in RCU list checking
Date: Tue, 14 Jan 2020 14:46:50 -0500	[thread overview]
Message-ID: <7959316e-7647-9ba3-5f1a-10c8d31a2994@intel.com> (raw)
In-Reply-To: <20200114194148.GD22037@ziepe.ca>

On 1/14/2020 2:41 PM, Jason Gunthorpe wrote:
> 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@gmail.com wrote:
>>>>> From: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.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@gmail.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.
>

Yep, seems like a bug to me. Patch forthcoming.

-Denny

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