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Subject: Re: [PATCH] rcu: update: Check rcu_bh_lock_map state in
 rcu_read_lock_bh_held
To:     paulmck@kernel.org
Cc:     josh@joshtriplett.org, rostedt@goodmis.org,
        mathieu.desnoyers@efficios.com, jiangshanlai@gmail.com,
        joel@joelfernandes.org, rcu@vger.kernel.org,
        linux-kernel@vger.kernel.org, urezki@gmail.com, frederic@kernel.org
References: <1624363521-19702-1-git-send-email-neeraju@codeaurora.org>
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 <61bed875-5ebf-03d8-58ea-e9263c534201@codeaurora.org>
 <20210622234652.GL4397@paulmck-ThinkPad-P17-Gen-1>
From:   Neeraj Upadhyay <neeraju@codeaurora.org>
Message-ID: <b37e3232-4151-e948-3987-b19c88f0e217@codeaurora.org>
Date:   Wed, 23 Jun 2021 09:27:11 +0530
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On 6/23/2021 5:16 AM, Paul E. McKenney wrote:
> On Wed, Jun 23, 2021 at 12:38:09AM +0530, Neeraj Upadhyay wrote:
>>
>>
>> On 6/22/2021 11:28 PM, Paul E. McKenney wrote:
>>> On Tue, Jun 22, 2021 at 05:35:21PM +0530, Neeraj Upadhyay wrote:
>>>> In addition to irq and softirq state, check rcu_bh_lock_map
>>>> state, to decide whether RCU bh lock is held.
>>>>
>>>> Signed-off-by: Neeraj Upadhyay <neeraju@codeaurora.org>
>>>
>>> My initial reaction was that "in_softirq() || irqs_disabled()" covers
>>> it because rcu_read_lock_bh() disables BH.  But you are right that it
>>> does seem a bit silly to ignore lockdep.
>>>
>>> So would it also make sense to have a WARN_ON_ONCE() if lockdep claims
>>> we are under rcu_read_lock_bh() protection, but "in_softirq() ||
>>> irqs_disabled()" think otherwise?
>>
>> After thinking more on this, looks like one intention of not
>> having lockdep check here was to catch scenarios where some code enables bh
>> after doing rcu_read_lock_bh(), as is mentioned in the comment above
>> rcu_read_lock_bh_held():
>>
>>    Note that if someone uses
>>    rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
>>    will show the situation.  This is useful for debug checks in functions
>>    that require that they be called within an RCU read-side critical
>>    section.
>>
>> Client users seem to be doing lockdep checks on returned value:
>> drivers/net/wireguard/peer.c
>>     RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held(),
>>
>> Similarly, any rcu_dereference_check(..., rcu_read_lock_bh_held()) usage
>> also triggers warning, if bh is enabled, inside rcu_read_lock_bh()
>> section.
>>
>> So, using 'in_softirq() || irqs_disabled()' condition looks to be sufficient
>> condition, to mark all read lock bh regions and adding '||
>> lock_is_held(&rcu_bh_lock_map)' to this condition does not seem to fit
>> well with the RCU_LOCKDEP_WARN(!rcu_read_lock_bh_held()) and
>> rcu_dereference_check(..., rcu_read_lock_bh_held()) calls, if we hit
>> the scenario, where bh lockmap state (shows bh lock acquired) conflicts with
>> the softirq/irq state .
> 
> That makes sense to me!
> 
> But should there be checks somewhere for something like
> "lock_is_held(&rcu_bh_lock_map) && !in_softirq() && !irqs_disabled()"?
> 
> 							Thanx, Paul
> 

I think this check is good to have inside rcu_read_lock_bh_held(), to
highlight this scenario explicitly; I am thinking, if it makes sense to
have lock_is_held(&rcu_bh_lock_map) check in rcu_softirq_qs() ?

Also, I think this check is more important for 
rcu_read_lock_sched_held(), where lockdep state is used as a sufficient 
condition, for marking a RCU
sched region. One more api is rcu_read_lock_any_held(), where we can
warn on conflicting cases.

   int rcu_read_lock_sched_held(void)
   {
         bool ret;

         if (rcu_read_lock_held_common(&ret))
                 return ret;
         return lock_is_held(&rcu_sched_lock_map) || !preemptible();
   }


Thanks
Neeraj

>> Thanks
>> Neeraj
>>
>>>> ---
>>>>    kernel/rcu/update.c | 2 +-
>>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
>>>> index c21b38c..d416f1c 100644
>>>> --- a/kernel/rcu/update.c
>>>> +++ b/kernel/rcu/update.c
>>>> @@ -333,7 +333,7 @@ int rcu_read_lock_bh_held(void)
>>>>    	if (rcu_read_lock_held_common(&ret))
>>>>    		return ret;
>>>> -	return in_softirq() || irqs_disabled();
>>>> +	return lock_is_held(&rcu_bh_lock_map) || in_softirq() || irqs_disabled();
>>>>    }
>>>>    EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);
>>>> -- 
>>>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
>>>> hosted by The Linux Foundation
>>>>
>>
>> -- 
>> QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a member of
>> the Code Aurora Forum, hosted by The Linux Foundation

-- 
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member of the Code Aurora Forum, hosted by The Linux Foundation