Re: [PATCH] scsi: Fix qemu boot hang problem

From: Hannes Reinecke
Date: Mon Aug 11 2014 - 02:16:44 EST

On 08/10/2014 02:54 PM, 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.

- 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@xxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxx>
Cc: Webb Scales <webbnh@xxxxxx>
Cc: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>
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);
- 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);

Reviewed-by: Hannes Reinecke <hare@xxxxxxx>


Dr. Hannes Reinecke zSeries & Storage
hare@xxxxxxx +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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