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From: Xiaoming Ni <nixiaoming@huawei.com>
To: Vasily Averin <vvs@virtuozzo.com>,
	"adobriyan@gmail.com" <adobriyan@gmail.com>,
	"akpm@linux-foundation.org" <akpm@linux-foundation.org>,
	"anna.schumaker@netapp.com" <anna.schumaker@netapp.com>,
	"arjan@linux.intel.com" <arjan@linux.intel.com>,
	"bfields@fieldses.org" <bfields@fieldses.org>,
	"chuck.lever@oracle.com" <chuck.lever@oracle.com>,
	"davem@davemloft.net" <davem@davemloft.net>,
	"gregkh@linuxfoundation.org" <gregkh@linuxfoundation.org>,
	"jlayton@kernel.org" <jlayton@kernel.org>,
	"luto@kernel.org" <luto@kernel.org>,
	"mingo@kernel.org" <mingo@kernel.org>,
	"Nadia.Derbey@bull.net" <Nadia.Derbey@bull.net>,
	"paulmck@linux.vnet.ibm.com" <paulmck@linux.vnet.ibm.com>,
	"semen.protsenko@linaro.org" <semen.protsenko@linaro.org>,
	"stable@kernel.org" <stable@kernel.org>,
	"stern@rowland.harvard.edu" <stern@rowland.harvard.edu>,
	"tglx@linutronix.de" <tglx@linutronix.de>,
	"torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"trond.myklebust@hammerspace.com"
	<trond.myklebust@hammerspace.com>,
	"viresh.kumar@linaro.org" <viresh.kumar@linaro.org>
Cc: "Huangjianhui (Alex)" <alex.huangjianhui@huawei.com>,
	Dailei <dylix.dailei@huawei.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>
Subject: Re: [PATCH v3 0/3] kernel/notifier.c: avoid duplicate registration
Date: Fri, 12 Jul 2019 21:11:57 +0800
Message-ID: <8ee6f763-ccce-ab58-3d96-21f5e1622916@huawei.com> (raw)
In-Reply-To: <d70ba831-85c7-d5a3-670a-144fa4d139cc@virtuozzo.com>

On 2019/7/11 21:57, Vasily Averin wrote:
> On 7/11/19 4:55 AM, Nixiaoming wrote:
>> On Wed, July 10, 2019 1:49 PM Vasily Averin wrote:
>>> On 7/10/19 6:09 AM, Xiaoming Ni wrote:
>>>> Registering the same notifier to a hook repeatedly can cause the hook
>>>> list to form a ring or lose other members of the list.
>>>
>>> I think is not enough to _prevent_ 2nd register attempt,
>>> it's enough to detect just attempt and generate warning to mark host in bad state.
>>>
>>
>> Duplicate registration is prevented in my patch, not just "mark host in bad state"
>>
>> Duplicate registration is checked and exited in notifier_chain_cond_register()
>>
>> Duplicate registration was checked in notifier_chain_register() but only 
>> the alarm was triggered without exiting. added by commit 831246570d34692e 
>> ("kernel/notifier.c: double register detection")
>>
>> My patch is like a combination of 831246570d34692e and notifier_chain_cond_register(),
>>  which triggers an alarm and exits when a duplicate registration is detected.
>>
>>> Unexpected 2nd register of the same hook most likely will lead to 2nd unregister,
>>> and it can lead to host crash in any time: 
>>> you can unregister notifier on first attempt it can be too early, it can be still in use.
>>> on the other hand you can never call 2nd unregister at all.
>>
>> Since the member was not added to the linked list at the time of the second registration, 
>> no linked list ring was formed. 
>> The member is released on the first unregistration and -ENOENT on the second unregistration.
>> After patching, the fault has been alleviated
> 
> You are wrong here.
> 2nd notifier's registration is a pure bug, this should never happen.
> If you know the way to reproduce this situation -- you need to fix it. 
> 
> 2nd registration can happen in 2 cases:
> 1) missed rollback, when someone forget to call unregister after successfull registration, 
> and then tried to call register again. It can lead to crash for example when according module will be unloaded.
> 2) some subsystem is registered twice, for example from  different namespaces.
> in this case unregister called during sybsystem cleanup in first namespace will incorrectly remove notifier used 
> in second namespace, it also can lead to unexpacted behaviour.
> 
So in these two cases, is it more reasonable to trigger BUG() directly when checking for duplicate registration ?
But why does current notifier_chain_register() just trigger WARN() without exiting ?
notifier_chain_cond_register() direct exit without triggering WARN() ?

Thanks

Xiaoming Ni

>> It may be more helpful to return an error code when someone tries to register the same
>> notification program a second time.
> 
> You are wrong again here, it is senseless.
> If you have detected 2nd register -- your node is already in bad state.
> 
>> But I noticed that notifier_chain_cond_register() returns 0 when duplicate registration 
>> is detected. At the same time, in all the existing export function comments of notify,
>> "Currently always returns zero"
>>
>> I am a bit confused: which is better?
>>
>>>
>>> Unfortunately I do not see any ways to handle such cases properly,
>>> and it seems for me your patches does not resolve this problem.
>>>
>>> Am I missed something probably?
>>>
>>>> case1: An infinite loop in notifier_chain_register() can cause soft lockup
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test1);
>>>>         atomic_notifier_chain_register(&test_notifier_list, &test2);
>>
>> Thanks
>>
>> Xiaoming Ni
>>
> 
> .
> 


  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-10  3:09 Xiaoming Ni
2019-07-10  5:49 ` Vasily Averin
2019-07-11  1:55   ` Nixiaoming
2019-07-11 13:57     ` Vasily Averin
2019-07-12 13:11       ` Xiaoming Ni [this message]
2019-07-12 14:07         ` gregkh
2019-07-14  2:45           ` Xiaoming Ni
2019-07-15  5:38             ` Vasily Averin
2019-07-16  2:00               ` Xiaoming Ni
2019-07-16 10:20                 ` Vasily Averin
2019-07-16 14:07                   ` Xiaoming Ni
2019-07-17 11:15                     ` Vasily Averin
2019-09-10  3:57                       ` Xiaoming Ni
2019-07-10  5:56 ` Greg KH
2019-07-11  1:32   ` Nixiaoming

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