linux-kernel - Re: [PATCH] block: aoenet: Replace GFP_ATOMIC with GFP_KERNEL in aoenet

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Message-ID: <b5e5e38f-5966-e68f-13a5-f9f0f68ef356@gmail.com>
Date:   Sun, 28 Jan 2018 10:12:51 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     Ed Cashin <ed@...hinworks.com>
Cc:     ed.cashin@....org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: aoenet: Replace GFP_ATOMIC with GFP_KERNEL in
 aoenet_rcv



On 2018/1/28 1:48, Ed Cashin wrote:
> If the tool cannot tell whether the protected state is manipulated by *another* piece of code called in atomic context, then it's insufficient.
>
>> On Jan 26, 2018, at 4:37 AM, Jia-Ju Bai <baijiaju1990@...il.com> wrote:
>>
>> After checking all possible call chains to aoenet_rcv(),
>> my tool finds that aoenet_rcv() is never called in atomic context,
>> namely never in an interrupt handler or holding a spinlock.
>> Thus GFP_ATOMIC is not necessary, and it can be replaced with GFP_KERNEL.
>>
>> This is found by a static analysis tool named DCNS written by myself.
>>
>> Signed-off-by: Jia-Ju Bai <baijiaju1990@...il.com>
>> ---
>> drivers/block/aoe/aoenet.c |    2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/aoe/aoenet.c b/drivers/block/aoe/aoenet.c
>> index 63773a9..d5fff7a 100644
>> --- a/drivers/block/aoe/aoenet.c
>> +++ b/drivers/block/aoe/aoenet.c
>> @@ -138,7 +138,7 @@ static int __init aoe_iflist_setup(char *str)
>>     if (dev_net(ifp) != &init_net)
>>         goto exit;
>>
>> -    skb = skb_share_check(skb, GFP_ATOMIC);
>> +    skb = skb_share_check(skb, GFP_KERNEL);
>>     if (skb == NULL)
>>         return 0;
>>     if (!is_aoe_netif(ifp))
>> -- 
>> 1.7.9.5
>>
>>

Sorry, I find my report is false positive after I manually check the code.
aoenet_rcv() is used as function pointer via "->func", and it is called 
in dev_queue_xmit_nit() in net/core/dev.c.
dev_queue_xmit_nit() calls a rcu_read_lock() before it calls 
pt_prev->func().
Thus it is right to use GFP_ATOMIC in aoenet_rcv().
Sorry again for my incorrect report...

Thanks,
Jia-Ju Bai