Re: 2.6.10-rc1-mm4

[Jens Axboe]

On Tue, Nov 09 2004, Jens Axboe wrote:
> On Tue, Nov 09 2004, Andrew Morton wrote:
> > Jens Axboe <axboe@xxxxxxx> wrote:
> > >
> > > On Tue, Nov 09 2004, Andrew Morton wrote:
> > > > +blk_sync_queue-updates.patch
> > > > 
> > > >  update an update to the md updates
> > > 
> > > I still don't think this is a good general export, it has very
> > > specialized use. For example, from the description it looks like this
> > > can be generally used on any block device and when it returns, we have
> > > synced the queue. This simply isn't true, there are absolutely no
> > > guarentees of that nature unless the block driver itself implements the
> > > __make_request() functionality and has taken proper precautions to
> > > prevent this already.
> > 
> > True.   So what do we do?  Grit our teeth and move it into MD?
> 
> That, or just comment it appropriately instead. Or, perhaps better, make

Here's a patch to just correct the comment. The md usage is safe because
it switches ->make_request_fn before syncing the queue and on second
thought it is a bit silly to add freeze/unfreeze functionality that no
one will use right now anyways. Alternatively, we could add this
functionality to the core so the interface would be

	/* callers wait for unfreeze */
	blk_freeze_queue(q, FREEZE_WAIT);

or

	/* callers are io errored immediately */
	blk_freeze_queue(q, FREEZE_END_IO);

which would work for both. I'll keep it in mind if such a use becomes
attractive, for now I think we should just correct the comment.

--- linux-2.6.10-rc1-mm4/drivers/block/ll_rw_blk.c~	2004-11-10 17:23:36.166564568 +0100
+++ linux-2.6.10-rc1-mm4/drivers/block/ll_rw_blk.c	2004-11-10 17:24:41.139041569 +0100
@@ -1361,7 +1361,7 @@
 EXPORT_SYMBOL(blk_stop_queue);
 
 /**
- * blk_sync_queue - cancel any pending callbacks a queue
+ * blk_sync_queue - cancel any pending callbacks on a queue
  * @q: the queue
  *
  * Description:
@@ -1369,7 +1369,9 @@
  *     on a queue, such as calling the unplug function after a timeout.
  *     A block device may call blk_sync_queue to ensure that any
  *     such activity is cancelled, thus allowing it to release resources
- *     the the callbacks might use.
+ *     the the callbacks might use. The caller must already have made sure
+ *     that its ->make_request_fn will not re-add plugging prior to calling
+ *     this function.
  *
  */
 void blk_sync_queue(struct request_queue *q)

-- 
Jens Axboe

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