linux-kernel - Re: [PATCH 3/4] [Patch-next] ACPI, APEI Fix the return value(==NULL) of acpi_pre

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Date:	Tue, 17 Aug 2010 13:47:40 +0900
From:	Jin Dongming <jin.dongming@...css.fujitsu.com>
To:	Huang Ying <ying.huang@...el.com>
CC:	Randy Dunlap <randy.dunlap@...cle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Andi Kleen <andi@...stfloor.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	ACPI <linux-acpi@...r.kernel.org>,
	LKLM <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/4] [Patch-next] ACPI, APEI Fix the return value(==NULL)
 of acpi_pre_map always.

(2010/08/17 13:15), Huang Ying wrote:
> On Tue, 2010-08-17 at 10:43 +0800, Jin Dongming wrote:
>> (2010/08/17 10:37), Huang Ying wrote:
>>> On Tue, 2010-08-17 at 08:56 +0800, Jin Dongming wrote:
>>>> acpi_pre_map() is used for remapping the physical address of I/O, so
>>>> it should be return NULL or remapped virtual address. The problem
>>>> is whether I/O remapping is successful or not, the function returns
>>>> NULL always.
>>>
>>> No. NULL is returned for error path only. Please check the code again.
>>
>> There three places used the local variable vaddr in acpi_pre_map() in next-tree.
>>    1. 115         vaddr = __acpi_try_ioremap(paddr, size);
>>    2. 122         vaddr = ioremap(pg_off, pg_sz);
>>    3. 135         vaddr = __acpi_try_ioremap(paddr, size);
>>
>> Let's think about the following statement.
>>    Assumption: the physical address has never been remapped.
>>    Step:
>>        1. vaddr == NULL
>>           Because the physical address is not registered in the acpi_iomaps,
>>           it should be returned NULL from __acpi_try_ioremap().
>>
>>        2. vaddr == the virtual address of the physical address.
>>           Here if ioremap is successful, the value of vaddr should be 
>>           the virtual address returned from ioremap().
>>
>>        3. vaddr == NULL                <== IMPORTANT
>>           Here it is because the physical address has not been registered
>>           in the acpi_iomaps yet, it still return NULL from __acpi_try_ioremap().
>>           So it is why vaddr == NULL, even if the physical address has never
>>           been remapped.
>>
>>     Result: vaddr == NULL.
> 
> return vaddr + (paddr - pg_off), is not NULL for common cases.
> 
>> And if the vaddr is not NULL, it could not be added into acpi_iomaps.
>> Codes in acpi_pre_map() is like following.
>>
>> 134         spin_lock_irqsave(&acpi_iomaps_lock, flags);
>> 135         vaddr = __acpi_try_ioremap(paddr, size);           <== the 3rd step
>> 136         if (vaddr) {
>> 137                 spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
>> 138                 iounmap(map->vaddr);
>> 139                 kfree(map);
>> 140                 return vaddr;
>> 141         }
>> 142         list_add_tail_rcu(&map->list, &acpi_iomaps);      <== add into acpi_iomaps.
>> 143         spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
> 
> Oops, this is a bug, and your patch really fix it. But I think you
> should change the patch description. NULL is not returned in almost all
> cases. Because (paddr - pg_off) is not zero in common cases.
> 
> Best Regards,
> Huang Ying
> 
OK. I will modify the description of this patch and resend it.

Best Regards,
Jin Dongming
> 
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