linux-kernel - Re: Why is (2 < 2) true? Is it a gcc bug?

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Date:	Fri, 17 Jan 2014 17:40:10 +0100
From:	Sebastian Riemer <sebastian.riemer@...fitbricks.com>
To:	"Dorau, Lukasz" <lukasz.dorau@...el.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: Re: Why is (2 < 2) true? Is it a gcc bug?

On 17.01.2014 14:55, Dorau, Lukasz wrote:
> On Friday, January 17, 2014 2:37 PM Dorau, Lukasz <lukasz.dorau@...el.com> wrote:
>>
>> Hi
>>
>> My story is very simply...
>> I applied the following patch:
>>
>> diff --git a/drivers/scsi/isci/init.c b/drivers/scsi/isci/init.c
>> --- a/drivers/scsi/isci/init.c
>> +++ b/drivers/scsi/isci/init.c
>> @@ -698,8 +698,11 @@ static int isci_pci_probe(struct pci_dev *pdev, const struct
>> pci_device_id *id)
>>  	if (err)
>>  		goto err_host_alloc;
>>
>> -	for_each_isci_host(i, isci_host, pdev)
>> +	for_each_isci_host(i, isci_host, pdev) {
>> +		pr_err("(%d < %d) == %d\n",\
>> +		       i, SCI_MAX_CONTROLLERS, (i < SCI_MAX_CONTROLLERS));
>>  		scsi_scan_host(to_shost(isci_host));
>> +	}
>>
>>  	return 0;
>>
>> --
>> 1.8.3.1
>>
>> Then I issued the command 'modprobe isci' on platform with two SCU controllers
>> (Patsburg D or T chipset)
>> and received the following, very strange, output:
>>
>> (0 < 2) == 1
>> (1 < 2) == 1
>> (2 < 2) == 1
>>
>> Can anyone explain why (2 < 2) is true? Is it a gcc bug?
>>
>> (The kernel was compiled using gcc version 4.8.2.)
>>
> 
> Some additional information:
> 
> The loop 'for' in macro ' for_each_isci_host ' defined as (drivers/scsi/isci/host.h:313):
> 
> #define for_each_isci_host(id, ihost, pdev) \
>         for (id = 0, ihost = to_pci_info(pdev)->hosts[id]; \
>              id < ARRAY_SIZE(to_pci_info(pdev)->hosts) && ihost; \
>              ihost = to_pci_info(pdev)->hosts[++id])
> 
> should be executed only for i = 0 and 1, because:
> ARRAY_SIZE(to_pci_info(pdev)->hosts) = SCI_MAX_CONTROLLERS = 2
> 
> but it was executed also for i=2 regardless the above loop's end condition. 

to_pci_info() can return NULL in dev_get_drvdata(). The use of
ARRAY_SIZE() is inappropriate.

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) +
__must_be_array(arr))

#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))

#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))

I would say that this was supposed to trigger a build error but it
didn't and added 1 to the loop end condition.

Can you test putting SCI_MAX_CONTROLLERS to the loop end condition, please?

Cheers,
Sebastian
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