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From: Dennis Zhou <dennisszhou@gmail.com>
To: Josef Bacik <josef@toxicpanda.com>
Cc: Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	kernel-team@fb.com, linux-block@vger.kernel.org,
	cgroups@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 04/15] blkcg: fix ref count issue with bio_blkcg using task_css
Date: Thu, 6 Sep 2018 11:21:23 -0400
Message-ID: <20180906152120.GA5055@dennisz-mbp.dhcp.thefacebook.com> (raw)
In-Reply-To: <20180831153538.brzgcm3rgmwfy3rg@destiny>

On Fri, Aug 31, 2018 at 11:35:39AM -0400, Josef Bacik wrote:
> On Thu, Aug 30, 2018 at 09:53:45PM -0400, Dennis Zhou wrote:
> > From: "Dennis Zhou (Facebook)" <dennisszhou@gmail.com>
> > +/**
> > + * blkcg_get_css - find and get a reference to the css
> > + *
> > + * Find the css associated with either the kthread or the current task.
> > + */
> > +static inline struct cgroup_subsys_state *blkcg_get_css(void)
> > +{
> > +	struct cgroup_subsys_state *css;
> > +
> > +	rcu_read_lock();
> > +
> > +	css = kthread_blkcg();
> > +	if (css) {
> > +		css_get(css);
> > +	} else {
> > +		while (true) {
> > +			css = task_css(current, io_cgrp_id);
> > +			if (likely(css_tryget(css)))
> > +				break;
> > +			cpu_relax();
> 
> Does this work?  I'm ignorant of what cpu_relax() does, but it seems if we're
> rcu_read_lock()'ed here we aren't going to queisce so if we fail to get the css
> here we just simply aren't going to get it unless we go to sleep right?  An
> honest question, because this is all magic to me, I'd like to understand how
> this isn't going to infinite loop on us if css_tryget(css) fails.
> 

Tejun replied earlier with an indepth answer. Thanks Tejun! I'll make
sure to add a comment detailing what's going on.

> > +/**
> > + * bio_blkcg - grab the blkcg associated with a bio
> > + * @bio: target bio
> > + *
> > + * This returns the blkcg associated with a bio, NULL if not associated.
> > + * Callers are expected to either handle NULL or know association has been
> > + * done prior to calling this.
> > + */
> >  static inline struct blkcg *bio_blkcg(struct bio *bio)
> >  {
> > -	struct cgroup_subsys_state *css;
> > -
> >  	if (bio && bio->bi_css)
> >  		return css_to_blkcg(bio->bi_css);
> > -	css = kthread_blkcg();
> > -	if (css)
> > -		return css_to_blkcg(css);
> > -	return css_to_blkcg(task_css(current, io_cgrp_id));
> > +	return NULL;
> >  }
> >  
> 
> So this is fine per se, but I know recently I was doing a bio_blkcg(NULL) to get
> whatever the blkcg was for the current task.  I threw that work away so I'm not
> worried about me, but have you made sure nobody else is doing something similar?
> 

Initially I thought the BFQ and CFQ stuff only interacted with bios
which should already be associated. Turns out during init, they rely on
that bio_blkcg to read from current and then do the wrong thing of hard
associating with it (_get vs _tryget).

I've created a __bio_blkcg which is identical to the old function with
notes to not use it. Making changes to BFQ and CFQ would take a good bit
more work to make sure I'm not breaking what they're expecting to do, so
I leave that to future work.

> >  static inline bool blk_cgroup_congested(void)
> > @@ -519,6 +549,11 @@ static inline struct request_list *blk_get_rl(struct request_queue *q,
> >  	rcu_read_lock();
> >  
> >  	blkcg = bio_blkcg(bio);
> > +	if (blkcg) {
> > +		css_get(&blkcg->css);
> > +	} else {
> > +		blkcg = css_to_blkcg(blkcg_get_css());
> > +	}
> 
> Kill these extra braces please.  Thanks,

Done.

Thanks,
Dennis

  parent reply index

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-08-31  1:53 [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Dennis Zhou
2018-08-31  1:53 ` [PATCH 01/15] Revert "blk-throttle: fix race between blkcg_bio_issue_check() and cgroup_rmdir()" Dennis Zhou
2018-08-31  1:53 ` [PATCH 02/15] blkcg: delay blkg destruction until after writeback has finished Dennis Zhou
2018-08-31 15:27   ` Josef Bacik
2018-08-31 20:19     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 03/15] blkcg: use tryget logic when associating a blkg with a bio Dennis Zhou
2018-08-31 15:30   ` Josef Bacik
2018-08-31 20:20     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 04/15] blkcg: fix ref count issue with bio_blkcg using task_css Dennis Zhou
2018-08-31 15:35   ` Josef Bacik
2018-08-31 23:04     ` Tejun Heo
2018-09-06 15:21     ` Dennis Zhou [this message]
2018-08-31  1:53 ` [PATCH 05/15] blkcg: update blkg_lookup_create to do locking Dennis Zhou
2018-08-31 15:37   ` Josef Bacik
2018-08-31 23:09   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 06/15] blkcg: always associate a bio with a blkg Dennis Zhou
2018-08-31  9:01   ` kbuild test robot
2018-08-31 10:02   ` kbuild test robot
2018-08-31 23:16   ` Tejun Heo
2018-09-06 20:41     ` Dennis Zhou
2018-09-07  3:03   ` [LKP] [blkcg] c02c58dab2: WARNING:at_block/blk-throttle.c:#blk_throtl_bio kernel test robot
2018-08-31  1:53 ` [PATCH 07/15] blkcg: consolidate bio_issue_init and blkg association Dennis Zhou
2018-08-31  9:19   ` kbuild test robot
2018-08-31 11:11   ` kbuild test robot
2018-08-31 15:42   ` Josef Bacik
2018-09-06 20:43     ` Dennis Zhou
2018-08-31 23:45   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 08/15] blkcg: associate a blkg for pages being evicted by swap Dennis Zhou
2018-08-31 15:44   ` Josef Bacik
2018-08-31 23:47   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 09/15] blkcg: associate writeback bios with a blkg Dennis Zhou
2018-08-31 15:45   ` Josef Bacik
2018-08-31 23:53   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 10/15] blkcg: remove bio->bi_css and instead use bio->bi_blkg Dennis Zhou
2018-08-31 15:46   ` Josef Bacik
2018-09-01  0:13   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 11/15] blkcg: remove additional reference to the css Dennis Zhou
2018-09-01  0:26   ` Tejun Heo
2018-09-06 20:45     ` Dennis Zhou
2018-08-31  1:53 ` [PATCH 12/15] blkcg: cleanup and make blk_get_rl use blkg_lookup_create Dennis Zhou
2018-09-01  0:29   ` Tejun Heo
2018-09-11  2:37   ` [LKP] [blkcg] 22f657e287: general_protection_fault:#[##] kernel test robot
2018-08-31  1:53 ` [PATCH 13/15] blkcg: change blkg reference counting to use percpu_ref Dennis Zhou
2018-08-31 15:49   ` Josef Bacik
2018-09-01  0:31   ` Tejun Heo
2018-09-06 20:46     ` Dennis Zhou
2018-09-07  3:08   ` [LKP] [blkcg] 6ef69a3a0b: WARNING:suspicious_RCU_usage kernel test robot
2018-08-31  1:53 ` [PATCH 14/15] blkcg: rename blkg_try_get to blkg_tryget Dennis Zhou
2018-08-31 15:50   ` Josef Bacik
2018-09-01  0:32   ` Tejun Heo
2018-08-31  1:53 ` [PATCH 15/15] blkcg: add average latency tracking to blk-cgroup Dennis Zhou
2018-08-31 10:22   ` kbuild test robot
2018-08-31 11:38   ` kbuild test robot
2018-09-01  0:35 ` [PATCH 00/15] blkcg ref count refactor/cleanup + blkcg avg_lat Tejun Heo

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