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Date: Fri, 17 Dec 2010 17:34:10 -0500
From: Vivek Goyal <vgoyal@...hat.com>
To: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc: linux-kernel@...r.kernel.org, jaxboe@...ionio.com, oleg@...hat.com
Subject: Re: [PATCH 2/2] blk-throttle: Some cleanups and race fixes in limit
update code
On Fri, Dec 17, 2010 at 02:28:05PM -0800, Paul E. McKenney wrote:
> On Wed, Dec 15, 2010 at 04:07:35PM -0500, Vivek Goyal wrote:
> > o When throttle group limits are updated through cgroups, a thread is woken
> > up to process these updates. While reviewing that code, oleg noted couple
> > of race conditions existed in the code and he also suggested that code can
> > be simplified.
> >
> > o This patch fixes the races simplifies the code based on Oleg's suggestions.
> > - Use xchg().
> > - Introduced a common function throtl_update_blkio_group_common()
> > which is shared now by all iops/bps update functions.
>
> OK, this looks good at the moment. The HW guys are still tearing their
> hair out, but this approach does appear to have an excellent chance of
> turning out to be safe. ;-)
>
> A few questions below, but assuming the answers turn out to be what
> I believe them to be:
>
> Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
Thanks a lot for the review paul. Yes all the assignments below without xchg()
are at initialization time and no other cpu is accessing it.
Thanks
Vivek
>
> > Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> > Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
> > ---
> > block/blk-throttle.c | 96 +++++++++++++++++++++-----------------------------
> > 1 files changed, 40 insertions(+), 56 deletions(-)
> >
> > diff --git a/block/blk-throttle.c b/block/blk-throttle.c
> > index 91bd444..549bc8e 100644
> > --- a/block/blk-throttle.c
> > +++ b/block/blk-throttle.c
> > @@ -97,7 +97,7 @@ struct throtl_data
> > /* Work for dispatching throttled bios */
> > struct delayed_work throtl_work;
> >
> > - atomic_t limits_changed;
> > + bool limits_changed;
> > };
> >
> > enum tg_state_flags {
> > @@ -188,6 +188,7 @@ static struct throtl_grp * throtl_find_alloc_tg(struct throtl_data *td,
> > RB_CLEAR_NODE(&tg->rb_node);
> > bio_list_init(&tg->bio_lists[0]);
> > bio_list_init(&tg->bio_lists[1]);
> > + td->limits_changed = false;
>
> This is at initialization time, correct? No other CPUs have access
> at this point, right?
>
> > /*
> > * Take the initial reference that will be released on destroy
> > @@ -725,34 +726,27 @@ static void throtl_process_limit_change(struct throtl_data *td)
> > struct throtl_grp *tg;
> > struct hlist_node *pos, *n;
> >
> > - if (!atomic_read(&td->limits_changed))
> > + if (!td->limits_changed)
> > return;
> >
> > - throtl_log(td, "limit changed =%d", atomic_read(&td->limits_changed));
> > + xchg(&td->limits_changed, false);
> >
> > - /*
> > - * Make sure updates from throtl_update_blkio_group_read_bps() group
> > - * of functions to tg->limits_changed are visible. We do not
> > - * want update td->limits_changed to be visible but update to
> > - * tg->limits_changed not being visible yet on this cpu. Hence
> > - * the read barrier.
> > - */
> > - smp_rmb();
> > + throtl_log(td, "limits changed");
> >
> > hlist_for_each_entry_safe(tg, pos, n, &td->tg_list, tg_node) {
> > - if (throtl_tg_on_rr(tg) && tg->limits_changed) {
> > - throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
> > - " riops=%u wiops=%u", tg->bps[READ],
> > - tg->bps[WRITE], tg->iops[READ],
> > - tg->iops[WRITE]);
> > + if (!tg->limits_changed)
> > + continue;
> > +
> > + if (!xchg(&tg->limits_changed, false))
> > + continue;
> > +
> > + throtl_log_tg(td, tg, "limit change rbps=%llu wbps=%llu"
> > + " riops=%u wiops=%u", tg->bps[READ], tg->bps[WRITE],
> > + tg->iops[READ], tg->iops[WRITE]);
> > +
> > + if (throtl_tg_on_rr(tg))
> > tg_update_disptime(td, tg);
> > - tg->limits_changed = false;
> > - }
> > }
> > -
> > - smp_mb__before_atomic_dec();
> > - atomic_dec(&td->limits_changed);
> > - smp_mb__after_atomic_dec();
> > }
> >
> > /* Dispatch throttled bios. Should be called without queue lock held. */
> > @@ -887,6 +881,15 @@ void throtl_unlink_blkio_group(void *key, struct blkio_group *blkg)
> > spin_unlock_irqrestore(td->queue->queue_lock, flags);
> > }
> >
> > +static void throtl_update_blkio_group_common(struct throtl_data *td,
> > + struct throtl_grp *tg)
> > +{
> > + xchg(&tg->limits_changed, true);
> > + xchg(&td->limits_changed, true);
> > + /* Schedule a work now to process the limit change */
> > + throtl_schedule_delayed_work(td->queue, 0);
> > +}
> > +
> > /*
> > * For all update functions, key should be a valid pointer because these
> > * update functions are called under blkcg_lock, that means, blkg is
> > @@ -900,61 +903,40 @@ static void throtl_update_blkio_group_read_bps(void *key,
> > struct blkio_group *blkg, u64 read_bps)
> > {
> > struct throtl_data *td = key;
> > + struct throtl_grp *tg = tg_of_blkg(blkg);
> >
> > - tg_of_blkg(blkg)->bps[READ] = read_bps;
> > - /* Make sure read_bps is updated before setting limits_changed */
> > - smp_wmb();
> > - tg_of_blkg(blkg)->limits_changed = true;
> > -
> > - /* Make sure tg->limits_changed is updated before td->limits_changed */
> > - smp_mb__before_atomic_inc();
> > - atomic_inc(&td->limits_changed);
> > - smp_mb__after_atomic_inc();
> > -
> > - /* Schedule a work now to process the limit change */
> > - throtl_schedule_delayed_work(td->queue, 0);
> > + tg->bps[READ] = read_bps;
> > + throtl_update_blkio_group_common(td, tg);
> > }
> >
> > static void throtl_update_blkio_group_write_bps(void *key,
> > struct blkio_group *blkg, u64 write_bps)
> > {
> > struct throtl_data *td = key;
> > + struct throtl_grp *tg = tg_of_blkg(blkg);
> >
> > - tg_of_blkg(blkg)->bps[WRITE] = write_bps;
> > - smp_wmb();
> > - tg_of_blkg(blkg)->limits_changed = true;
> > - smp_mb__before_atomic_inc();
> > - atomic_inc(&td->limits_changed);
> > - smp_mb__after_atomic_inc();
> > - throtl_schedule_delayed_work(td->queue, 0);
> > + tg->bps[WRITE] = write_bps;
> > + throtl_update_blkio_group_common(td, tg);
> > }
> >
> > static void throtl_update_blkio_group_read_iops(void *key,
> > struct blkio_group *blkg, unsigned int read_iops)
> > {
> > struct throtl_data *td = key;
> > + struct throtl_grp *tg = tg_of_blkg(blkg);
> >
> > - tg_of_blkg(blkg)->iops[READ] = read_iops;
> > - smp_wmb();
> > - tg_of_blkg(blkg)->limits_changed = true;
> > - smp_mb__before_atomic_inc();
> > - atomic_inc(&td->limits_changed);
> > - smp_mb__after_atomic_inc();
> > - throtl_schedule_delayed_work(td->queue, 0);
> > + tg->iops[READ] = read_iops;
> > + throtl_update_blkio_group_common(td, tg);
> > }
> >
> > static void throtl_update_blkio_group_write_iops(void *key,
> > struct blkio_group *blkg, unsigned int write_iops)
> > {
> > struct throtl_data *td = key;
> > + struct throtl_grp *tg = tg_of_blkg(blkg);
> >
> > - tg_of_blkg(blkg)->iops[WRITE] = write_iops;
> > - smp_wmb();
> > - tg_of_blkg(blkg)->limits_changed = true;
> > - smp_mb__before_atomic_inc();
> > - atomic_inc(&td->limits_changed);
> > - smp_mb__after_atomic_inc();
> > - throtl_schedule_delayed_work(td->queue, 0);
> > + tg->iops[WRITE] = write_iops;
> > + throtl_update_blkio_group_common(td, tg);
> > }
> >
> > void throtl_shutdown_timer_wq(struct request_queue *q)
> > @@ -1001,6 +983,7 @@ int blk_throtl_bio(struct request_queue *q, struct bio **biop)
> > */
> > update_disptime = false;
> > goto queue_bio;
> > +
> > }
> >
> > /* Bio is with-in rate limit of group */
> > @@ -1041,7 +1024,7 @@ int blk_throtl_init(struct request_queue *q)
> >
> > INIT_HLIST_HEAD(&td->tg_list);
> > td->tg_service_tree = THROTL_RB_ROOT;
> > - atomic_set(&td->limits_changed, 0);
> > + td->limits_changed = false;
>
> And the name leads me to believe that there are no other CPUs accessing
> things at this point as well. Correct?
>
> > /* Init root group */
> > tg = &td->root_tg;
> > @@ -1053,6 +1036,7 @@ int blk_throtl_init(struct request_queue *q)
> > /* Practically unlimited BW */
> > tg->bps[0] = tg->bps[1] = -1;
> > tg->iops[0] = tg->iops[1] = -1;
> > + td->limits_changed = false;
>
> Ditto?
>
> > /*
> > * Set root group reference to 2. One reference will be dropped when
> > --
> > 1.7.1
> >
--
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