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SubjectRe: [PATCH RESEND] remove the queue unlock in scsi_requset_fn
On 08/16/12 07:52, Bart Van Assche wrote:
> On 08/16/12 01:35, Chanho Min wrote:
>>> functions will occur in line. I also don't see why the sdev reference
>>> couldn't drop to zero here.
>> scsi_request_fn is called under the lock of request_queue->queue_lock.
>> If we drop the sdev reference to zero here,
>> scsi_device_dev_release_usercontext is
>> invoked and make request_queue to NULL. When caller of scsi_request_fn try to
>> unlock request_queue->queue_lock, the oops is occurred.
>
> Whether or not your patch is applied, if the put_device() call in
> scsi_request_fn() decreases the sdev reference count to zero, the
> scsi_request_fn() caller will still try to unlock the queue lock after
> scsi_request_fn() finished and hence will trigger a use-after-free. I'm
> afraid the only real solution is to modify the SCSI and/or block layers
> such that scsi_remove_device() can't finish while scsi_request_fn() is
> in progress. And once that is guaranteed the get_device() / put_device()
> pair can be left out from scsi_request_fn().

(replying to my own e-mail)

How about the patch below ?

[PATCH] Fix device removal race

If the put_device() call in scsi_request_fn() drops the sdev refcount
to zero then the spin_lock_irq() call after the put_device() call
triggers a use-after-free. Avoid that by making sure that blk_cleanup_queue()
can only finish after all active scsi_request_fn() calls have returned.
---
block/blk-core.c | 1 +
drivers/scsi/scsi_lib.c | 10 ++--------
include/linux/blkdev.h | 5 +++++
3 files changed, 8 insertions(+), 8 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index 4b4dbdf..0e4da3b 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -388,6 +388,7 @@ void blk_drain_queue(struct request_queue *q, bool drain_all)
__blk_run_queue(q);

drain |= q->nr_rqs_elvpriv;
+ drain |= q->request_fn_active;

/*
* Unfortunately, requests are queued at and tracked from
diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index ffd7773..10bb348 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1514,9 +1514,7 @@ static void scsi_request_fn(struct request_queue *q)
struct scsi_cmnd *cmd;
struct request *req;

- if(!get_device(&sdev->sdev_gendev))
- /* We must be tearing the block queue down already */
- return;
+ q->request_fn_active++;

/*
* To start with, we keep looping until the queue is empty, or until
@@ -1626,11 +1624,7 @@ out_delay:
if (sdev->device_busy == 0)
blk_delay_queue(q, SCSI_QUEUE_DELAY);
out:
- /* must be careful here...if we trigger the ->remove() function
- * we cannot be holding the q lock */
- spin_unlock_irq(q->queue_lock);
- put_device(&sdev->sdev_gendev);
- spin_lock_irq(q->queue_lock);
+ q->request_fn_active--;
}

u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 4e72a9d..11c1987 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -377,6 +377,11 @@ struct request_queue {

unsigned int nr_sorted;
unsigned int in_flight[2];
+ /*
+ * Number of active request_fn() calls for those request_fn()
+ * implementations that unlock the queue_lock, e.g. scsi_request_fn().
+ */
+ unsigned int request_fn_active;

unsigned int rq_timeout;
struct timer_list timeout;
--
1.7.7



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