linux-kernel - Re: [PATCH 1/2] [Patch-next] ACPI, APEI Fix the return value(==NULL) of acpi_pre

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Date:	Fri, 03 Sep 2010 17:37:22 +0900
From:	Jin Dongming <jin.dongming@...css.fujitsu.com>
To:	Bjorn Helgaas <bjorn.helgaas@...com>
CC:	APEI-lenb <lenb@...nel.org>, Huang Ying <ying.huang@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Randy Dunlap <randy.dunlap@...cle.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	ACPI <linux-acpi@...r.kernel.org>,
	LKLM <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] [Patch-next] ACPI, APEI Fix the return value(==NULL)
 of acpi_pre_map().

Hi, Bjorn Helgaas

I will modify this patch and resend it as soon as possible in next week.

Best Regards,
Jin Dongming

(2010/09/03 0:47), Bjorn Helgaas wrote:
> On Thursday, September 02, 2010 01:20:18 am 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
>> that when (paddr - pg_off) equals 0, no matter whether I/O remapping is
>> successful or not, the function returns NULL.
>>
>> In acpi_pre_map(), after the physical address is remapped successfully,
>> it will check whether the physical address has been added into acpi_iomaps
>> list again.
>>    - If the physical address has been added into acpi_iomaps, the virtual
>>      address will be saved in vaddr.
>>    - Otherwise, NULL be saved in vaddr.
>>
>> So if the physical address is the first time to be remapped, vaddr will be
>> NULL always.
>>
>> In many cases, (paddr - pg_off) may not equal 0. So the function could work
>> well. In our machine, (paddr - pg_off) equals 0, so the return value of
>> acpi_pre_map() is NULL.
>>
>> This patch fixed it and I confirmed it on x86_64 next-tree.
> 
> This description is complicated and obscures the very simple bug
> you are fixing.  I suggest something like this:
> 
>   ACPI: fix acpi_pre_map() return value
> 
>   After we ioremap() a new region, we call __acpi_try_ioremap() to
>   see whether another thread has already mapped the same region.
>   This check clobbers "vaddr", so compute the return value using
>   the ioremap() result "map->vaddr" instead.
> 
>> v2:
>>     Modified the unsuitable description of patch.
>>
>> v3:
>>     Modified the wrong words in the description of patch.   
>>
>> Signed-off-by: Jin Dongming <jin.dongming@...css.fujitsu.com>
>> Acked-by: Huang Ying <ying.huang@...el.com>
>> ---
>>  drivers/acpi/atomicio.c |    4 ++--
>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/acpi/atomicio.c b/drivers/acpi/atomicio.c
>> index 8f8bd73..1bc2614 100644
>> --- a/drivers/acpi/atomicio.c
>> +++ b/drivers/acpi/atomicio.c
>> @@ -133,7 +133,7 @@ static void __iomem *acpi_pre_map(phys_addr_t paddr,
>>  
>>  	spin_lock_irqsave(&acpi_iomaps_lock, flags);
>>  	vaddr = __acpi_try_ioremap(paddr, size);
>> -	if (vaddr) {
>> +	if (unlikely(vaddr)) {
> 
> This "unlikely" addition also obscures things.  It's completely
> unrelated to the bug you're fixing, so it should be in a different
> patch, and I'm dubious that there's enough performance advantage to
> compensate for the decreased code readability.  I wouldn't make this
> change unless you can measure a performance improvement.
> 
>>  		spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
>>  		iounmap(map->vaddr);
>>  		kfree(map);
>> @@ -142,7 +142,7 @@ static void __iomem *acpi_pre_map(phys_addr_t paddr,
>>  	list_add_tail_rcu(&map->list, &acpi_iomaps);
>>  	spin_unlock_irqrestore(&acpi_iomaps_lock, flags);
>>  
>> -	return vaddr + (paddr - pg_off);
>> +	return map->vaddr + (paddr - pg_off);
> 
> Since you're changing this line anyway, I think you should change it to:
> 
>   return map->vaddr + (paddr - map->paddr);
> 
> so it matches __acpi_try_ioremap().
> 
>>  err_unmap:
>>  	iounmap(vaddr);
>>  	return NULL;
>> -- 1.7.1.
>>
>> --
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> 


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