James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 2008-07-02 at 20:45 +0200, Jens Axboe wrote: >> On Wed, Jul 02 2008, James Bottomley wrote: > >> > On Wed, 2008-07-02 at 13:50 +0200, Jens Axboe wrote: >> > > On Wed, Jul 02 2008, Elias Oltmanns wrote: >> > > > > The blk_plug_queue change looks reasonable ... however, blk_plug_queue >> > > > > itself looks like it might not entirely need the queue lock ... I need >> > > > > to investigate more closely. >> > > > >> > > > Well, I rather think it does. We have to serialise access to the >> > > > unplug_timer and there is a call to __set_bit() which, as I understand, >> > > > requires the calling function to ensure atomicity. >> > > >> > > Yep, blk_plug_device() needs to be called with the queue lock held. >> > >> > That's what the comment says ... but if you replaced the test_bit with >> > an atomic operation then the rest of it does look to be in no need of >> > serialisation ... unless there's something I missed? >> >> Indeed, but then you would have to use atomic bitops everywhere and that >> is the bit we moved away from. > > Not necessarily ... only for QUEUE_FLAG_CLUSTER. That's really only in > this one place and then the one in blk_remove_plug would have to become > test_and_clear_bit. All the other places barring loop_unplug() are only > tests (which don't affect the atomicity). > > It's just for SCSI the double spin lock followed by double spin unlock > to get the locking right is kind of nasty ... I'm just wondering what > the universe would look like if it were rendered unnecessary. We have to consider one more thing: Without the locking in blk_plug_device(), the following sequence of events may occur: 1. Some calls blk_plug_device(). 2. Someone else calls blk_stop_queue() right after the check in blk_plug_device() has been performed but before the test_and_set_bit() has been called. 3. The unplug_timer expires, __generic_unplug_device() discovers that the queue has been stopped in the meantime and returns without removing the plug. 4. Someone calls blk_start_queue() later on which will execute the ->request_fn() even though blk_queue_plugged() is still true. In order to resolve this, we'd have to switch the calls to blk_remove_plug() and blk_queue_stopped() in __generic_unplug_device(). I'm not quite sure at the moment whether this would have any further implications. Regards, Elias -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
