On Tue, Aug 26, 2014 at 08:56:36PM +0900, Toshiaki Makita wrote: > cfq_group_service_tree_add() is applying new_weight at the beginning of > the function via cfq_update_group_weight(). > This actually allows weight to change between adding it to and subtracting > it from children_weight, and triggers WARN_ON_ONCE() in > cfq_group_service_tree_del(), or even causes oops by divide error during > vfr calculation in cfq_group_service_tree_add(). > > The detailed scenario is as follows: > 1. Create blkio cgroups X and Y as a child of X. This is a bit confusing. Maybe use a different letter for the parent X? > Set X's weight to 500 and perform some I/O to apply new_weight. > This X's I/O completes before starting Y's I/O. > 2. Y starts I/O and cfq_group_service_tree_add() is called with Y. > 3. cfq_group_service_tree_add() walks up the tree during children_weight > calculation and adds parent X's weight (500) to children_weight of root. > children_weight becomes 500. > 4. Set X's weight to 1000. > 5. X starts I/O and cfq_group_service_tree_add() is called with X. > 6. cfq_group_service_tree_add() applies its new_weight (1000). > 7. I/O of Y completes and cfq_group_service_tree_del() is called with Y. > 8. I/O of X completes and cfq_group_service_tree_del() is called with X. > 9. cfq_group_service_tree_del() subtracts X's weight (1000) from > children_weight of root. children_weight becomes -500. > This triggers WARN_ON_ONCE(). > 10. Set X's weight to 500. > 11. X starts I/O and cfq_group_service_tree_add() is called with X. > 12. cfq_group_service_tree_add() applies its new_weight (500) and adds it > to children_weight of root. children_weight becomes 0. Calcularion of > vfr triggers oops by divide error. > > weight should be updated right before adding it to children_weight. > > Reported-by: Ruki Sekiya <sekiya.ruki@xxxxxxxxxxxxx> > Signed-off-by: Toshiaki Makita <makita.toshiaki@xxxxxxxxxxxxx> Acked-by: Tejun Heo <tj@xxxxxxxxxx> Comment in the code explaining that new_weight application has to happen only on activation would be nice tho. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
