Messages in this thread | | | Date | Thu, 17 Apr 2008 09:13:35 +0200 | From | Jens Axboe <> | Subject | Re: Block: Prevent busy looping |
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On Thu, Apr 17 2008, Elias Oltmanns wrote: > Jens Axboe <jens.axboe@oracle.com> wrote: > > On Wed, Apr 16 2008, Elias Oltmanns wrote: > >> blk_run_queue() as well as blk_start_queue() plug the device on reentry > >> and schedule blk_unplug_work() right afterwards. However, > >> blk_plug_device() takes care of that already and makes sure that there is > >> a short delay before blk_unplug_work() is scheduled. This is important > >> to prevent busy looping and possibly system lockups as observed here: > >> <http://permalink.gmane.org/gmane.linux.ide/28351>. > > > > If you call blk_start_queue() and blk_run_queue(), you better mean it. > > There should be no delay. The only reason it does blk_plug_device() is > > so that the work queue function will actually do some work. > > Well, I'm mainly concerned with blk_run_queue(). In a comment it says > that it should recurse only once so as not to overrun the stack. On my > machine, however, immediate rescheduling may have exactly as disastrous > consequences as an overrunning stack would have since the system locks > up completely. > > Just to get this straight: Are low level drivers allowed to rely on > blk_run_queue() that there will be no loops or do they have to make sure > that this function is not called from the request_fn() of the same > queue?
It's not really designed for being called recursively. Which isn't the problem imo, the problem is SCSI apparently being dumb and calling blk_run_queue() all the time. blk_run_queue() must run the queue NOW. If SCSI wants something like 'run the queue in a bit', it should use blk_plug_device() instead.
> > In the newer kernels we just do: > > > > set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags); > > kblockd_schedule_work(q, &q->unplug_work); > > > > instead, which is much better. > > Only as long as it doesn't get called from the request_fn() of the same > queue. Otherwise, there may be no chance for other threads to clear the > condition that caused blk_run_queue() to be called in the first place.
Broken usage.
-- Jens Axboe
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