Messages in this thread | | | Subject | Re: [PATCH] blk-cgroup: check blkcg policy is enabled in blkg_create() | From | "yukuai (C)" <> | Date | Tue, 12 Oct 2021 09:09:03 +0800 |
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On 2021/10/11 23:23, Michal Koutný wrote: > Hello. > > On Fri, Oct 08, 2021 at 03:27:20PM +0800, Yu Kuai <yukuai3@huawei.com> wrote: >> This is because blkg_alloc() is called from blkg_conf_prep() without >> holding 'q->queue_lock', and elevator is exited before blkg_create(): > > IIUC the problematic interleaving is this one (I've noticed `blkg->pd[i] > = NULL` to thread 2 call trace):
The new blkg will not add to blkg_list untill pd_init_fn() is done in blkg_create(), thus blkcg_deactivate_policy() can't access this blkg. > >> thread 1 thread 2 >> blkg_conf_prep >> spin_lock_irq(&q->queue_lock); >> blkg_lookup_check -> return NULL >> spin_unlock_irq(&q->queue_lock); >> >> blkg_alloc >> blkcg_policy_enabled -> true >> pd = ->pd_alloc_fn >> blk_mq_exit_sched >> bfq_exit_queue >> blkcg_deactivate_policy >> spin_lock_irq(&q->queue_lock); >> __clear_bit(pol->plid, q->blkcg_pols); >> > pol->pd_free_fn(blkg->pd[i]); > blkg->pd[i] = NULL; >> >> spin_unlock_irq(&q->queue_lock); >> q->elevator = NULL; > blkg->pd[i] = pd >> spin_lock_irq(&q->queue_lock); >> blkg_create >> if (blkg->pd[i]) >> ->pd_init_fn -> q->elevator is NULL >> spin_unlock_irq(&q->queue_lock); > > In high-level terms, is this a race between (blk)io controller attribute > write and a device scheduler (elevator) switch? > If so, I'd add it to the commit message. > >> Fix the problem by checking that policy is still enabled in >> blkg_create(). > > Is this sufficient wrt some other q->elevator users later? > >> @@ -252,6 +266,9 @@ static struct blkcg_gq *blkg_create(struct blkcg *blkcg, >> goto err_free_blkg; >> } >> > > I'd add a comment here like: > >> Re-check policies are still enabled, since the caller blkg_conf_prep() >> temporarily drops q->queue_lock and we can race with >> blk_mq_exit_sched() removing policies.
Thanks for your advice.
Best regards, Kuai > >> + if (new_blkg) >> + blkg_check_pd(q, new_blkg); >> + > > Thanks, > Michal > . >
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