linux-kernel - Re: [PATCH kernel] srcu: Fix static initialization

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Date:   Tue, 22 Sep 2020 10:41:37 +1000
From:   Alexey Kardashevskiy <aik@...abs.ru>
To:     paulmck@...nel.org
Cc:     rcu@...r.kernel.org, Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH kernel] srcu: Fix static initialization



On 17/09/2020 02:12, Paul E. McKenney wrote:
> On Fri, Sep 11, 2020 at 06:52:08AM -0700, Paul E. McKenney wrote:
>> On Fri, Sep 11, 2020 at 03:09:41PM +1000, Alexey Kardashevskiy wrote:
>>> On 11/09/2020 04:53, Paul E. McKenney wrote:
>>>> On Wed, Sep 09, 2020 at 10:31:03PM +1000, Alexey Kardashevskiy wrote:
> 
> [ . . . ]
> 
>>>>> init_srcu_struct_nodes() assumes ssp->sda!=NULL but alloc_percpu() fails
>>>>> here:
>>>>>
>>>>> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/mm/percpu.c#n1734
>>>>> ===
>>>>> 	} else if (mutex_lock_killable(&pcpu_alloc_mutex)) {
>>>>> 			pcpu_memcg_post_alloc_hook(objcg, NULL, 0, size);
>>>>> 			return NULL;
>>>>> ===
>>>>>
>>>>> I am still up to reading that osr-rcuusage.pdf to provide better
>>>>> analysis :) Thanks,
>>>>
>>>> Ah, got it!  Does the following patch help?
>>>>
>>>> There will likely also need to be changes to cleanup_srcu_struct(),
>>>> but first let's see if I understand the problem.  ;-)
>>>>
>>>> 							Thanx, Paul
>>>>
>>>> ------------------------------------------------------------------------
>>>>
>>>> diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
>>>> index c13348e..6f7880a 100644
>>>> --- a/kernel/rcu/srcutree.c
>>>> +++ b/kernel/rcu/srcutree.c
>>>> @@ -177,11 +177,13 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
>>>>  	INIT_DELAYED_WORK(&ssp->work, process_srcu);
>>>>  	if (!is_static)
>>>>  		ssp->sda = alloc_percpu(struct srcu_data);
>>>> +	if (!ssp->sda)
>>>> +		return -ENOMEM;
>>>>  	init_srcu_struct_nodes(ssp, is_static);
>>>>  	ssp->srcu_gp_seq_needed_exp = 0;
>>>>  	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
>>>>  	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
>>>
>>> The line above confuses me a bit. What you propose returns without
>>> smp_store_release() called which should not matter I suppose.
>>
>> The idea is that if init_srcu_struct() returns -ENOMEM, the structure
>> has not been initialized and had better not be used.  If the calling code
>> cannot handle that outcome, then the calling code needs to do something
>> to insulate init_srcu_struct() from signals.  One thing that it could
>> do would be to invoke init_srcu_struct() from a workqueue handler and
>> wait for this handler to complete.
>>
>> Please keep in mind that there is nothing init_srcu_struct() can do
>> about this:  The srcu_struct is useless unless alloc_percpu() succeeds.
>>
>> And yes, I do need to update the header comments to make this clear.
>>
>>> Otherwise it should work, although I cannot verify right now as my box
>>> went down and since it is across Pacific - it may take time to power
>>> cycle it :) Thanks,
>>
>> I know that feeling!  And here is hoping that the box is out of reach
>> of the local hot spots.  ;-)
> 
> Just following up...  Did that patch help?


Yes it did.

Tested-by: Alexey Kardashevskiy <aik@...abs.ru>



> 
> 							Thanx, Paul
> 

-- 
Alexey