linux-kernel - Re: [PATCH] scsi: ibmvscsi: Don't use rc uninitialized in ibmvscsi_do

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Message-ID: <031eaca1-bb6d-a14f-bb66-a520219549e4@linux.vnet.ibm.com>
Date:   Mon, 3 Jun 2019 16:30:25 -0700
From:   Tyrel Datwyler <tyreld@...ux.vnet.ibm.com>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Nathan Chancellor <natechancellor@...il.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>
Cc:     clang-built-linux@...glegroups.com, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org
Subject: Re: [PATCH] scsi: ibmvscsi: Don't use rc uninitialized in
 ibmvscsi_do_work

On 06/02/2019 03:15 AM, Michael Ellerman wrote:
> Hi Nathan,
> 
> Nathan Chancellor <natechancellor@...il.com> writes:
>> clang warns:
>>
>> drivers/scsi/ibmvscsi/ibmvscsi.c:2126:7: warning: variable 'rc' is used
>> uninitialized whenever switch case is taken [-Wsometimes-uninitialized]
>>         case IBMVSCSI_HOST_ACTION_NONE:
>>              ^~~~~~~~~~~~~~~~~~~~~~~~~
>> drivers/scsi/ibmvscsi/ibmvscsi.c:2151:6: note: uninitialized use occurs
>> here
>>         if (rc) {
>>             ^~
>>
>> Initialize rc to zero so that the atomic_set and dev_err statement don't
>> trigger for the cases that just break.
>>
>> Fixes: 035a3c4046b5 ("scsi: ibmvscsi: redo driver work thread to use enum action states")
>> Link: https://github.com/ClangBuiltLinux/linux/issues/502
>> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
>> ---
>>  drivers/scsi/ibmvscsi/ibmvscsi.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
>> index 727c31dc11a0..6714d8043e62 100644
>> --- a/drivers/scsi/ibmvscsi/ibmvscsi.c
>> +++ b/drivers/scsi/ibmvscsi/ibmvscsi.c
>> @@ -2118,7 +2118,7 @@ static unsigned long ibmvscsi_get_desired_dma(struct vio_dev *vdev)
>>  static void ibmvscsi_do_work(struct ibmvscsi_host_data *hostdata)
>>  {
>>  	unsigned long flags;
>> -	int rc;
>> +	int rc = 0;
>>  	char *action = "reset";
>>  
>>  	spin_lock_irqsave(hostdata->host->host_lock, flags);
> 
> It's always preferable IMHO to keep any initialisation as localised as
> possible, so that the compiler can continue to warn about uninitialised
> usages elsewhere. In this case that would mean doing the rc = 0 in the
> switch, something like:
> 
> diff --git a/drivers/scsi/ibmvscsi/ibmvscsi.c b/drivers/scsi/ibmvscsi/ibmvscsi.c
> index 727c31dc11a0..7ee5755cf636 100644
> --- a/drivers/scsi/ibmvscsi/ibmvscsi.c
> +++ b/drivers/scsi/ibmvscsi/ibmvscsi.c
> @@ -2123,9 +2123,6 @@ static void ibmvscsi_do_work(struct ibmvscsi_host_data *hostdata)
>  
>         spin_lock_irqsave(hostdata->host->host_lock, flags);
>         switch (hostdata->action) {
> -       case IBMVSCSI_HOST_ACTION_NONE:
> -       case IBMVSCSI_HOST_ACTION_UNBLOCK:
> -               break;
>         case IBMVSCSI_HOST_ACTION_RESET:
>                 spin_unlock_irqrestore(hostdata->host->host_lock, flags);
>                 rc = ibmvscsi_reset_crq_queue(&hostdata->queue, hostdata);
> @@ -2142,7 +2139,10 @@ static void ibmvscsi_do_work(struct ibmvscsi_host_data *hostdata)
>                 if (!rc)
>                         rc = ibmvscsi_send_crq(hostdata, 0xC001000000000000LL, 0);
>                 break;
> +       case IBMVSCSI_HOST_ACTION_NONE:
> +       case IBMVSCSI_HOST_ACTION_UNBLOCK:
>         default:
> +               rc = 0;
>                 break;
>         }
> 
> 
> But then that makes me wonder if that's actually correct?
> 
> If we get an action that we don't recognise should we just throw it away
> like that? (by doing hostdata->action = IBMVSCSI_HOST_ACTION_NONE). Tyrel?

On initial pass I was ok with this, but after thinking on it I think it is more
subtle.

The right approach is to set rc = 0 for HOST_ACTION_UNBLOCK as we want to fall
through. For HOST_ACTION_NONE and default we need to return directly from the
function.

-Tyrel

> 
> cheers
>