iocost: fix infinite loop bug in adjust_inuse_and_calc_cost()
authorTejun Heo <tj@kernel.org>
Mon, 14 Sep 2020 15:05:13 +0000 (11:05 -0400)
committerJens Axboe <axboe@kernel.dk>
Mon, 14 Sep 2020 23:25:39 +0000 (17:25 -0600)
commitaa67db24b6761277d731cc3434420283479927ca
treed0f2619c33c90a1f333b3d67e65a4f23d133cf91
parent769b628de0b9a7ab0b81e6eecca5ee7bf66e978d
iocost: fix infinite loop bug in adjust_inuse_and_calc_cost()

adjust_inuse_and_calc_cost() is responsible for reducing the amount of
donated weights dynamically in period as the budget runs low. Because we
don't want to do full donation calculation in period, we keep latching up
inuse by INUSE_ADJ_STEP_PCT of the active weight of the cgroup until the
resulting hweight_inuse is satisfactory.

Unfortunately, the adj_step calculation was reading the active weight before
acquiring ioc->lock. Because the current thread could have lost race to
activate the iocg to another thread before entering this function, it may
read the active weight as zero before acquiring ioc->lock. When this
happens, the adj_step is calculated as zero and the incremental adjustment
loop becomes an infinite one.

Fix it by fetching the active weight after acquiring ioc->lock.

Fixes: b0853ab4a238 ("blk-iocost: revamp in-period donation snapbacks")
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
block/blk-iocost.c