Tejun Heo 0b80f9866e iocost: protect iocg->abs_vdebt with iocg->waitq.lock ... abs_vdebt is an atomic_64 which tracks how much over budget a given cgroup is and controls the activation of use_delay mechanism. Once a cgroup goes over budget from forced IOs, it has to pay it back with its future budget. The progress guarantee on debt paying comes from the iocg being active - active iocgs are processed by the periodic timer, which ensures that as time passes the debts dissipate and the iocg returns to normal operation. However, both iocg activation and vdebt handling are asynchronous and a sequence like the following may happen. 1. The iocg is in the process of being deactivated by the periodic timer. 2. A bio enters ioc_rqos_throttle(), calls iocg_activate() which returns without anything because it still sees that the iocg is already active. 3. The iocg is deactivated. 4. The bio from #2 is over budget but needs to be forced. It increases abs_vdebt and goes over the threshold and enables use_delay. 5. IO control is enabled for the iocg's subtree and now IOs are attributed to the descendant cgroups and the iocg itself no longer issues IOs. This leaves the iocg with stuck abs_vdebt - it has debt but inactive and no further IOs which can activate it. This can end up unduly punishing all the descendants cgroups. The usual throttling path has the same issue - the iocg must be active while throttled to ensure that future event will wake it up - and solves the problem by synchronizing the throttling path with a spinlock. abs_vdebt handling is another form of overage handling and shares a lot of characteristics including the fact that it isn't in the hottest path. This patch fixes the above and other possible races by strictly synchronizing abs_vdebt and use_delay handling with iocg->waitq.lock. Signed-off-by: Tejun Heo <tj@kernel.org> Reported-by: Vlad Dmitriev <vvd@fb.com> Cc: stable@vger.kernel.org # v5.4+ Fixes: e1518f63f2 ("blk-iocost: Don't let merges push vtime into the future") Signed-off-by: Jens Axboe <axboe@kernel.dk>		2020-05-05 09:23:18 -06:00
..
.gitignore	.gitignore: add SPDX License Identifier	2020-03-25 11:50:48 +01:00
cgroup_event_listener.c
iocost_coef_gen.py	blkcg: add tools/cgroup/iocost_coef_gen.py	2019-08-28 21:17:17 -06:00
iocost_monitor.py	iocost: protect iocg->abs_vdebt with iocg->waitq.lock	2020-05-05 09:23:18 -06:00
Makefile	tools: fix cross-compile var clobbering	2018-02-21 15:35:42 -08:00