James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > On Thu, 2008-07-03 at 13:24 +0200, Jens Axboe wrote: >> On Thu, Jul 03 2008, Elias Oltmanns wrote: > >> > Elias Oltmanns <eo@xxxxxxxxxxxxxx> wrote: >> > > 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: >> > >>> > > 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: >> > >> > Actually, it's worse than that. Locking is required in order to make >> > absolutely sure that the unplug_timer is active iff QUEUE_FLAG_PLUGGED >> > is set. Admittedly, it seems *very* unlikely that blk_remove_plug() will >> > complete before the call to mod_timer() in blk_plug_device() even though >> > it has started only *after* a call to test_and_set_bit(). However, if >> > such a thing would ever happen, it could have dire consequences. >> >> Both are races possible without either atomic bitops or the queue lock >> being held. We can't properly mix eg set_bit() and __set_bit(). The >> plugged bit is the most hammered, so it's staying non-atomic and SCSI >> will need to provide proper locking there. > > You're the boss. > > Actually, after all of this, it looks like the host queue plug is > superfluous. If the host actually says not ready from > scsi_host_queue_ready() we go to the not ready processing clause in > scsi_prep_fn() which actually checks the outstanding on the current > device and plugs the queue if there aren't any commands. This is > actually more correct behaviour than a blind plug regardless (and it's > also done under the queue lock), so I think this is the correct fix. Indeed, a very neat way out. Will this be queued up for stable too? 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
