linux-kernel - Re: [PATCH] scsi: Fix qemu boot hang problem

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Date:	Tue, 19 Aug 2014 10:31:41 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Guenter Roeck <linux@...ck-us.net>
Cc:	"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>, Jens Axboe <axboe@...nel.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] scsi: Fix qemu boot hang problem

James, can you please send this on ASAP?  Sitting for oever a week
on a boot regression that comes with a patch isn't reasonable.

On Sun, Aug 10, 2014 at 05:54:25AM -0700, 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);
>  }
>  
> -- 
> 1.9.1
> 
> --
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