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Date:	Mon, 11 Aug 2014 08:33:38 -0700
From:	Venkatesh Srinivas <venkateshs@...gle.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Webb Scales <webbnh@...com>
Subject: Re: [PATCH] scsi: Fix qemu boot hang problem

Should we wrap the sdev->device_busy read and test in a new
scsi_device_busy(), so as to preserve the old polarity?

Reviewed-by: Venkatesh Srinivas <venkateshs@...gle.com>

On 8/11/14, Jens Axboe <axboe@...nel.dk> wrote:
> On 2014-08-10 06:54, Guenter Roeck wrote:
>> The latest kernel fails to boot qemu arm images when using scsi
>> for disk access. Boot gets stuck after the following messages.
>>
>> brd: module loaded
>> sym53c8xx 0000:00:0c.0: enabling device (0100 -> 0103)
>> sym0: <895a> rev 0x0 at pci 0000:00:0c.0 irq 93
>> sym0: No NVRAM, ID 7, Fast-40, LVD, parity checking
>> sym0: SCSI BUS has been reset.
>> scsi host0: sym-2.2.3
>>
>> Bisect points to commit 71e75c97f97a ("scsi: convert device_busy to
>> atomic_t"). Code inspection shows the following suspicious change
>> in scsi_request_fn.
>>
>> out_delay:
>> -       if (sdev->device_busy == 0 && !scsi_device_blocked(sdev))
>> +       if (atomic_read(&sdev->device_busy) &&
>> !scsi_device_blocked(sdev))
>> 		blk_delay_queue(q, SCSI_QUEUE_DELAY);
>> 	}
>>
>> 'sdev->device_busy == 0' was replaced with
>> 'atomic_read(&sdev->device_busy)',
>> meaning the logic was reversed. Changing this expression to
>> '!atomic_read(&sdev->device_busy)' fixes the problem.
>>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Hannes Reinecke <hare@...e.de>
>> Cc: Webb Scales <webbnh@...com>
>> Cc: Jens Axboe <axboe@...nel.dk>
>> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
>> ---
>>   drivers/scsi/scsi_lib.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
>> index 9c44392..ce62e87 100644
>> --- a/drivers/scsi/scsi_lib.c
>> +++ b/drivers/scsi/scsi_lib.c
>> @@ -1774,7 +1774,7 @@ static void scsi_request_fn(struct request_queue
>> *q)
>>   	blk_requeue_request(q, req);
>>   	atomic_dec(&sdev->device_busy);
>>   out_delay:
>> -	if (atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
>> +	if (!atomic_read(&sdev->device_busy) && !scsi_device_blocked(sdev))
>>   		blk_delay_queue(q, SCSI_QUEUE_DELAY);
>>   }
>>
>>
>
> That's a no-brainer obvious fix!
>
> Acked-by: Jens Axboe <axboe@...com>
>
>
> --
> Jens Axboe
>
> --
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