From: Tejun Heo tj@kernel.org
commit d16baa3f1453c14d680c5fee01cd122a22d0e0ce upstream.
When initializing iocost for a queue, its rqos should be registered before the blkcg policy is activated to allow policy data initiailization to lookup the associated ioc. This unfortunately means that the rqos methods can be called on bios before iocgs are attached to all existing blkgs.
While the race is theoretically possible on ioc_rqos_throttle(), it mostly happened in ioc_rqos_merge() due to the difference in how they lookup ioc. The former determines it from the passed in @rqos and then bails before dereferencing iocg if the looked up ioc is disabled, which most likely is the case if initialization is still in progress. The latter looked up ioc by dereferencing the possibly NULL iocg making it a lot more prone to actually triggering the bug.
* Make ioc_rqos_merge() use the same method as ioc_rqos_throttle() to look up ioc for consistency.
* Make ioc_rqos_throttle() and ioc_rqos_merge() test for NULL iocg before dereferencing it.
* Explain the danger of NULL iocgs in blk_iocost_init().
Signed-off-by: Tejun Heo tj@kernel.org Reported-by: Jonathan Lemon bsd@fb.com Cc: stable@vger.kernel.org # v5.4+ Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- block/blk-iocost.c | 16 +++++++++++----- 1 file changed, 11 insertions(+), 5 deletions(-)
--- a/block/blk-iocost.c +++ b/block/blk-iocost.c @@ -2525,8 +2525,8 @@ static void ioc_rqos_throttle(struct rq_ bool use_debt, ioc_locked; unsigned long flags;
- /* bypass IOs if disabled or for root cgroup */ - if (!ioc->enabled || !iocg->level) + /* bypass IOs if disabled, still initializing, or for root cgroup */ + if (!ioc->enabled || !iocg || !iocg->level) return;
/* calculate the absolute vtime cost */ @@ -2653,14 +2653,14 @@ static void ioc_rqos_merge(struct rq_qos struct bio *bio) { struct ioc_gq *iocg = blkg_to_iocg(bio->bi_blkg); - struct ioc *ioc = iocg->ioc; + struct ioc *ioc = rqos_to_ioc(rqos); sector_t bio_end = bio_end_sector(bio); struct ioc_now now; u64 vtime, abs_cost, cost; unsigned long flags;
- /* bypass if disabled or for root cgroup */ - if (!ioc->enabled || !iocg->level) + /* bypass if disabled, still initializing, or for root cgroup */ + if (!ioc->enabled || !iocg || !iocg->level) return;
abs_cost = calc_vtime_cost(bio, iocg, true); @@ -2837,6 +2837,12 @@ static int blk_iocost_init(struct reques ioc_refresh_params(ioc, true); spin_unlock_irq(&ioc->lock);
+ /* + * rqos must be added before activation to allow iocg_pd_init() to + * lookup the ioc from q. This means that the rqos methods may get + * called before policy activation completion, can't assume that the + * target bio has an iocg associated and need to test for NULL iocg. + */ rq_qos_add(q, rqos); ret = blkcg_activate_policy(q, &blkcg_policy_iocost); if (ret) {