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block, bfq: access and cache blkg data only when safe
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In blk-cgroup, operations on blkg objects are protected with the
request_queue lock. This is no more the lock that protects
I/O-scheduler operations in blk-mq. In fact, the latter are now
protected with a finer-grained per-scheduler-instance lock. As a
consequence, although blkg lookups are also rcu-protected, blk-mq I/O
schedulers may see inconsistent data when they access blkg and
blkg-related objects. BFQ does access these objects, and does incur
this problem, in the following case.

The blkg_lookup performed in bfq_get_queue, being protected (only)
through rcu, may happen to return the address of a copy of the
original blkg. If this is the case, then the blkg_get performed in
bfq_get_queue, to pin down the blkg, is useless: it does not prevent
blk-cgroup code from destroying both the original blkg and all objects
directly or indirectly referred by the copy of the blkg. BFQ accesses
these objects, which typically causes a crash for NULL-pointer
dereference of memory-protection violation.

Some additional protection mechanism should be added to blk-cgroup to
address this issue. In the meantime, this commit provides a quick
temporary fix for BFQ: cache (when safe) blkg data that might
disappear right after a blkg_lookup.

In particular, this commit exploits the following facts to achieve its
goal without introducing further locks.  Destroy operations on a blkg
invoke, as a first step, hooks of the scheduler associated with the
blkg. And these hooks are executed with bfqd->lock held for BFQ. As a
consequence, for any blkg associated with the request queue an
instance of BFQ is attached to, we are guaranteed that such a blkg is
not destroyed, and that all the pointers it contains are consistent,
while that instance is holding its bfqd->lock. A blkg_lookup performed
with bfqd->lock held then returns a fully consistent blkg, which
remains consistent until this lock is held. In more detail, this holds
even if the returned blkg is a copy of the original one.

Finally, also the object describing a group inside BFQ needs to be
protected from destruction on the blkg_free of the original blkg
(which invokes bfq_pd_free). This commit adds private refcounting for
this object, to let it disappear only after no bfq_queue refers to it
any longer.

This commit also removes or updates some stale comments on locking
issues related to blk-cgroup operations.

Reported-by: Tomas Konir <[email protected]>
Reported-by: Lee Tibbert <[email protected]>
Reported-by: Marco Piazza <[email protected]>
Signed-off-by: Paolo Valente <[email protected]>
Tested-by: Tomas Konir <[email protected]>
Tested-by: Lee Tibbert <[email protected]>
Tested-by: Marco Piazza <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
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@Algodev-github @axboe
Algodev-github authored and axboe committed Jun 8, 2017
1 parent 85d0331 commit 8f9bebc
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Showing 3 changed files with 105 additions and 36 deletions.
116 changes: 93 additions & 23 deletions block/bfq-cgroup.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -52,7 +52,7 @@ BFQG_FLAG_FNS(idling)
BFQG_FLAG_FNS(empty)
#undef BFQG_FLAG_FNS

/* This should be called with the queue_lock held. */
/* This should be called with the scheduler lock held. */
static void bfqg_stats_update_group_wait_time(struct bfqg_stats *stats)
{
unsigned long long now;
Expand All @@ -67,7 +67,7 @@ static void bfqg_stats_update_group_wait_time(struct bfqg_stats *stats)
bfqg_stats_clear_waiting(stats);
}

/* This should be called with the queue_lock held. */
/* This should be called with the scheduler lock held. */
static void bfqg_stats_set_start_group_wait_time(struct bfq_group *bfqg,
struct bfq_group *curr_bfqg)
{
Expand All @@ -81,7 +81,7 @@ static void bfqg_stats_set_start_group_wait_time(struct bfq_group *bfqg,
bfqg_stats_mark_waiting(stats);
}

/* This should be called with the queue_lock held. */
/* This should be called with the scheduler lock held. */
static void bfqg_stats_end_empty_time(struct bfqg_stats *stats)
{
unsigned long long now;
Expand Down Expand Up @@ -203,12 +203,30 @@ struct bfq_group *bfqq_group(struct bfq_queue *bfqq)

static void bfqg_get(struct bfq_group *bfqg)
{
return blkg_get(bfqg_to_blkg(bfqg));
bfqg->ref++;
}

void bfqg_put(struct bfq_group *bfqg)
{
return blkg_put(bfqg_to_blkg(bfqg));
bfqg->ref--;

if (bfqg->ref == 0)
kfree(bfqg);
}

static void bfqg_and_blkg_get(struct bfq_group *bfqg)
{
/* see comments in bfq_bic_update_cgroup for why refcounting bfqg */
bfqg_get(bfqg);

blkg_get(bfqg_to_blkg(bfqg));
}

void bfqg_and_blkg_put(struct bfq_group *bfqg)
{
bfqg_put(bfqg);

blkg_put(bfqg_to_blkg(bfqg));
}

void bfqg_stats_update_io_add(struct bfq_group *bfqg, struct bfq_queue *bfqq,
Expand Down Expand Up @@ -312,7 +330,11 @@ void bfq_init_entity(struct bfq_entity *entity, struct bfq_group *bfqg)
if (bfqq) {
bfqq->ioprio = bfqq->new_ioprio;
bfqq->ioprio_class = bfqq->new_ioprio_class;
bfqg_get(bfqg);
/*
* Make sure that bfqg and its associated blkg do not
* disappear before entity.
*/
bfqg_and_blkg_get(bfqg);
}
entity->parent = bfqg->my_entity; /* NULL for root group */
entity->sched_data = &bfqg->sched_data;
Expand Down Expand Up @@ -399,6 +421,8 @@ struct blkg_policy_data *bfq_pd_alloc(gfp_t gfp, int node)
return NULL;
}

/* see comments in bfq_bic_update_cgroup for why refcounting */
bfqg_get(bfqg);
return &bfqg->pd;
}

Expand Down Expand Up @@ -426,7 +450,7 @@ void bfq_pd_free(struct blkg_policy_data *pd)
struct bfq_group *bfqg = pd_to_bfqg(pd);

bfqg_stats_exit(&bfqg->stats);
return kfree(bfqg);
bfqg_put(bfqg);
}

void bfq_pd_reset_stats(struct blkg_policy_data *pd)
Expand Down Expand Up @@ -496,9 +520,10 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
* Move @bfqq to @bfqg, deactivating it from its old group and reactivating
* it on the new one. Avoid putting the entity on the old group idle tree.
*
* Must be called under the queue lock; the cgroup owning @bfqg must
* not disappear (by now this just means that we are called under
* rcu_read_lock()).
* Must be called under the scheduler lock, to make sure that the blkg
* owning @bfqg does not disappear (see comments in
* bfq_bic_update_cgroup on guaranteeing the consistency of blkg
* objects).
*/
void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
struct bfq_group *bfqg)
Expand All @@ -519,16 +544,12 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
bfq_deactivate_bfqq(bfqd, bfqq, false, false);
else if (entity->on_st)
bfq_put_idle_entity(bfq_entity_service_tree(entity), entity);
bfqg_put(bfqq_group(bfqq));
bfqg_and_blkg_put(bfqq_group(bfqq));

/*
* Here we use a reference to bfqg. We don't need a refcounter
* as the cgroup reference will not be dropped, so that its
* destroy() callback will not be invoked.
*/
entity->parent = bfqg->my_entity;
entity->sched_data = &bfqg->sched_data;
bfqg_get(bfqg);
/* pin down bfqg and its associated blkg */
bfqg_and_blkg_get(bfqg);

if (bfq_bfqq_busy(bfqq)) {
bfq_pos_tree_add_move(bfqd, bfqq);
Expand All @@ -545,8 +566,9 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
* @bic: the bic to move.
* @blkcg: the blk-cgroup to move to.
*
* Move bic to blkcg, assuming that bfqd->queue is locked; the caller
* has to make sure that the reference to cgroup is valid across the call.
* Move bic to blkcg, assuming that bfqd->lock is held; which makes
* sure that the reference to cgroup is valid across the call (see
* comments in bfq_bic_update_cgroup on this issue)
*
* NOTE: an alternative approach might have been to store the current
* cgroup in bfqq and getting a reference to it, reducing the lookup
Expand Down Expand Up @@ -604,6 +626,57 @@ void bfq_bic_update_cgroup(struct bfq_io_cq *bic, struct bio *bio)
goto out;

bfqg = __bfq_bic_change_cgroup(bfqd, bic, bio_blkcg(bio));
/*
* Update blkg_path for bfq_log_* functions. We cache this
* path, and update it here, for the following
* reasons. Operations on blkg objects in blk-cgroup are
* protected with the request_queue lock, and not with the
* lock that protects the instances of this scheduler
* (bfqd->lock). This exposes BFQ to the following sort of
* race.
*
* The blkg_lookup performed in bfq_get_queue, protected
* through rcu, may happen to return the address of a copy of
* the original blkg. If this is the case, then the
* bfqg_and_blkg_get performed in bfq_get_queue, to pin down
* the blkg, is useless: it does not prevent blk-cgroup code
* from destroying both the original blkg and all objects
* directly or indirectly referred by the copy of the
* blkg.
*
* On the bright side, destroy operations on a blkg invoke, as
* a first step, hooks of the scheduler associated with the
* blkg. And these hooks are executed with bfqd->lock held for
* BFQ. As a consequence, for any blkg associated with the
* request queue this instance of the scheduler is attached
* to, we are guaranteed that such a blkg is not destroyed, and
* that all the pointers it contains are consistent, while we
* are holding bfqd->lock. A blkg_lookup performed with
* bfqd->lock held then returns a fully consistent blkg, which
* remains consistent until this lock is held.
*
* Thanks to the last fact, and to the fact that: (1) bfqg has
* been obtained through a blkg_lookup in the above
* assignment, and (2) bfqd->lock is being held, here we can
* safely use the policy data for the involved blkg (i.e., the
* field bfqg->pd) to get to the blkg associated with bfqg,
* and then we can safely use any field of blkg. After we
* release bfqd->lock, even just getting blkg through this
* bfqg may cause dangling references to be traversed, as
* bfqg->pd may not exist any more.
*
* In view of the above facts, here we cache, in the bfqg, any
* blkg data we may need for this bic, and for its associated
* bfq_queue. As of now, we need to cache only the path of the
* blkg, which is used in the bfq_log_* functions.
*
* Finally, note that bfqg itself needs to be protected from
* destruction on the blkg_free of the original blkg (which
* invokes bfq_pd_free). We use an additional private
* refcounter for bfqg, to let it disappear only after no
* bfq_queue refers to it any longer.
*/
blkg_path(bfqg_to_blkg(bfqg), bfqg->blkg_path, sizeof(bfqg->blkg_path));
bic->blkcg_serial_nr = serial_nr;
out:
rcu_read_unlock();
Expand Down Expand Up @@ -640,8 +713,6 @@ static void bfq_reparent_leaf_entity(struct bfq_data *bfqd,
* @bfqd: the device data structure with the root group.
* @bfqg: the group to move from.
* @st: the service tree with the entities.
*
* Needs queue_lock to be taken and reference to be valid over the call.
*/
static void bfq_reparent_active_entities(struct bfq_data *bfqd,
struct bfq_group *bfqg,
Expand Down Expand Up @@ -692,8 +763,7 @@ void bfq_pd_offline(struct blkg_policy_data *pd)
/*
* The idle tree may still contain bfq_queues belonging
* to exited task because they never migrated to a different
* cgroup from the one being destroyed now. No one else
* can access them so it's safe to act without any lock.
* cgroup from the one being destroyed now.
*/
bfq_flush_idle_tree(st);

Expand Down
2 changes: 1 addition & 1 deletion block/bfq-iosched.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -3665,7 +3665,7 @@ void bfq_put_queue(struct bfq_queue *bfqq)

kmem_cache_free(bfq_pool, bfqq);
#ifdef CONFIG_BFQ_GROUP_IOSCHED
bfqg_put(bfqg);
bfqg_and_blkg_put(bfqg);
#endif
}

Expand Down
23 changes: 11 additions & 12 deletions block/bfq-iosched.h
View file
Original file line number Diff line number Diff line change
Expand Up @@ -759,6 +759,12 @@ struct bfq_group {
/* must be the first member */
struct blkg_policy_data pd;

/* cached path for this blkg (see comments in bfq_bic_update_cgroup) */
char blkg_path[128];

/* reference counter (see comments in bfq_bic_update_cgroup) */
int ref;

struct bfq_entity entity;
struct bfq_sched_data sched_data;

Expand Down Expand Up @@ -838,7 +844,7 @@ struct bfq_group *bfq_find_set_group(struct bfq_data *bfqd,
struct blkcg_gq *bfqg_to_blkg(struct bfq_group *bfqg);
struct bfq_group *bfqq_group(struct bfq_queue *bfqq);
struct bfq_group *bfq_create_group_hierarchy(struct bfq_data *bfqd, int node);
void bfqg_put(struct bfq_group *bfqg);
void bfqg_and_blkg_put(struct bfq_group *bfqg);

#ifdef CONFIG_BFQ_GROUP_IOSCHED
extern struct cftype bfq_blkcg_legacy_files[];
Expand Down Expand Up @@ -910,20 +916,13 @@ void bfq_add_bfqq_busy(struct bfq_data *bfqd, struct bfq_queue *bfqq);
struct bfq_group *bfqq_group(struct bfq_queue *bfqq);

#define bfq_log_bfqq(bfqd, bfqq, fmt, args...) do { \
char __pbuf[128]; \
\
blkg_path(bfqg_to_blkg(bfqq_group(bfqq)), __pbuf, sizeof(__pbuf)); \
blk_add_trace_msg((bfqd)->queue, "bfq%d%c %s " fmt, (bfqq)->pid, \
blk_add_trace_msg((bfqd)->queue, "bfq%d%c %s " fmt, (bfqq)->pid,\
bfq_bfqq_sync((bfqq)) ? 'S' : 'A', \
__pbuf, ##args); \
bfqq_group(bfqq)->blkg_path, ##args); \
} while (0)

#define bfq_log_bfqg(bfqd, bfqg, fmt, args...) do { \
char __pbuf[128]; \
\
blkg_path(bfqg_to_blkg(bfqg), __pbuf, sizeof(__pbuf)); \
blk_add_trace_msg((bfqd)->queue, "%s " fmt, __pbuf, ##args); \
} while (0)
#define bfq_log_bfqg(bfqd, bfqg, fmt, args...) \
blk_add_trace_msg((bfqd)->queue, "%s " fmt, (bfqg)->blkg_path, ##args)

#else /* CONFIG_BFQ_GROUP_IOSCHED */

Expand Down

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