On Tuesday 28 August 2007 02:35, Evgeniy Polyakov wrote: > On Mon, Aug 27, 2007 at 02:57:37PM -0700, Daniel Phillips (phillips@xxxxxxxxx) wrote: > > Say Evgeniy, something I was curious about but forgot to ask you > > earlier... > > > > On Wednesday 08 August 2007 03:17, Evgeniy Polyakov wrote: > > > ...All oerations are not atomic, since we do not care about > > > precise number of bios, but a fact, that we are close or close > > > enough to the limit. > > > ... in bio->endio > > > + q->bio_queued--; > > > > In your proposed patch, what prevents the race: > > > > cpu1 cpu2 > > > > read q->bio_queued > > q->bio_queued-- > > write q->bio_queued - 1 > > Whoops! We leaked a throttle count. > > We do not care about one cpu being able to increase its counter > higher than the limit, such inaccuracy (maximum bios in flight thus > can be more than limit, difference is equal to the number of CPUs - > 1) is a price for removing atomic operation. I thought I pointed it > in the original description, but might forget, that if it will be an > issue, that atomic operations can be introduced there. Any > uber-precise measurements in the case when we are close to the edge > will not give us any benefit at all, since were are already in the > grey area. This is not just inaccurate, it is suicide. Keep leaking throttle counts and eventually all of them will be gone. No more IO on that block device! > Another possibility is to create a queue/device pointer in the bio > structure to hold original device and then in its backing dev > structure add a callback to recalculate the limit, but it increases > the size of the bio. Do we need this? Different issue. Yes, I think we need a nice simple approach like that, and prove it is stable before worrying about the size cost. Regards, Daniel - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
