On 06/24/13 17:38, James Bottomley wrote:
I really don't like this because it's shuffling potentially fragile
lifetime rules since you now have to have the sdev deleted from the
starved list before final put. That becomes an unstated assumption
within the code.
The theory is that the starved list processing may be racing with a
scsi_remove_device, so when we unlock the host lock, the device (and the
queue) may be destroyed. OK, so I agree with this, remote a possibility
though it may be. The easy way of fixing it without making assumptions
is this, isn't it? All it requires is that the queue be destroyed after
the starved list entry is deleted in the sdev release code.
James
---
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 86d5220..f294cc6 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -434,6 +434,7 @@ static void scsi_run_queue(struct request_queue *q)
list_splice_init(&shost->starved_list, &starved_list);
while (!list_empty(&starved_list)) {
+ struct request_queue *slq;
/*
* As long as shost is accepting commands and we have
* starved queues, call blk_run_queue. scsi_request_fn
@@ -456,10 +457,21 @@ static void scsi_run_queue(struct request_queue *q)
continue;
}
+ /*
+ * once we drop the host lock, a racing scsi_remove_device may
+ * remove the sdev from the starved list and destroy it and
+ * the queue. Mitigate by taking a reference to the queue and
+ * never touching the sdev again after we drop the host lock.
+ */
+ slq = sdev->request_queue;
+ if (!blk_get_queue(slq))
+ continue;
+
spin_unlock(shost->host_lock);
- spin_lock(sdev->request_queue->queue_lock);
- __blk_run_queue(sdev->request_queue);
- spin_unlock(sdev->request_queue->queue_lock);
+
+ blk_run_queue(slq);
+ blk_put_queue(slq);
+
spin_lock(shost->host_lock);
}
/* put any unprocessed entries back */
Since the above patch invokes blk_put_queue() with interrupts disabled
it may cause blk_release_queue() to be invoked with interrupts disabled.
Sorry but I'm not sure whether that will work fine.
Bart.
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