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Date: Tue, 14 Nov 2006 12:41:20 +0100 From: Jens Axboe <jens.axboe@...cle.com> To: Pierre Ossman <drzeus-list@...eus.cx> Cc: LKML <linux-kernel@...r.kernel.org> Subject: Re: How to cleanly shut down a block device On Tue, Nov 14 2006, Pierre Ossman wrote: > Russell King wrote: > > Just arrange for the mmc_queue_thread() to empty the queue when > > MMC_QUEUE_EXIT is set, and then exit. I thought this was something > > that the block layer looked after (Jens must have missed this in his > > original review of the MMC code.) > > > > mmc_queue_thread() will empty the thread when MMC_QUEUE_EXIT is set. The > problem is that we do not set that bit until the last person closes the > device. In order to avoid problems we need to empty the queue before > mmc_blk_remove() exits (after which the card structure is no longer valid). > > > The handling of userspace keeping the device open despite the hardware > > having been removed is already in place. > > > > > > Ok, that's one less problem for me to worry about. :) > > Jens Axboe wrote: > > What do you mean by "killing off the queue"? As long as the queue can be > > gotten at, it needs to remain valid. That is what the references are > > for. > > > > I do: > > del_gendisk(); > (wait for queue to become empty, i.e. elv_next_request() == NULL) > blk_cleanup_queue(); elv_next_request() returning NULL means nothing wrt the queue being empty. > and then assume that the request function will no longer be called for > this queue. > > Suggested patch: I think you are making this way too complicated, it's actually pretty simple: you call blk_put_queue() or blk_cleanup_queue() (same thing) when _you_ drop your reference to the queue. That's just normal cleanup. When a device goes away, you make sure that you know about this. I said that SCSI clears q->queuedata, so it knows that when ->request_fn is invoked with a NULL q->queuedata (where it stores the device pointer), the device is not there and the request should just be flushed to heaven. Don't make any assumptions about when request_fn will be called or not. That's bound to be racy anyway. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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