From: Ming Lei <ming.lei@xxxxxxxxxx>
The kernfs built-in lock of 'kn->count' is held in sysfs .show/.store
path. Meantime, inside block's .show/.store callback, q->sysfs_lock is
required.
However, when mq & iosched kobjects are removed via
blk_mq_unregister_dev() & elv_unregister_queue(), q->sysfs_lock is held
too. This way causes AB-BA lock because the kernfs built-in lock of
'kn-count' is required inside kobject_del() too, see the lockdep warning[1].
On the other hand, it isn't necessary to acquire q->sysfs_lock for
both blk_mq_unregister_dev() & elv_unregister_queue() because
clearing REGISTERED flag prevents storing to 'queue/scheduler'
from being happened. Also sysfs write(store) is exclusive, so no
necessary to hold the lock for elv_unregister_queue() when it is
called in switching elevator path.
So split .sysfs_lock into two: one is still named as .sysfs_lock for
covering sync .store, the other one is named as .sysfs_dir_lock
for covering kobjects and related status change.
sysfs itself can handle the race between add/remove kobjects and
showing/storing attributes under kobjects. For switching scheduler
via storing to 'queue/scheduler', we use the queue flag of
QUEUE_FLAG_REGISTERED with .sysfs_lock for avoiding the race, then
we can avoid to hold .sysfs_lock during removing/adding kobjects.
[1] lockdep warning
======================================================
WARNING: possible circular locking dependency detected
5.3.0-rc3-00044-g73277fc75ea0 #1380 Not tainted
------------------------------------------------------
rmmod/777 is trying to acquire lock:
00000000ac50e981 (kn->count#202){++++}, at: kernfs_remove_by_name_ns+0x59/0x72
but task is already holding lock:
00000000fb16ae21 (&q->sysfs_lock){+.+.}, at: blk_unregister_queue+0x78/0x10b
which lock already depends on the new lock.
the existing dependency chain (in reverse order) is:
-> #1 (&q->sysfs_lock){+.+.}:
__lock_acquire+0x95f/0xa2f
lock_acquire+0x1b4/0x1e8
__mutex_lock+0x14a/0xa9b
blk_mq_hw_sysfs_show+0x63/0xb6
sysfs_kf_seq_show+0x11f/0x196
seq_read+0x2cd/0x5f2
vfs_read+0xc7/0x18c
ksys_read+0xc4/0x13e
do_syscall_64+0xa7/0x295
entry_SYSCALL_64_after_hwframe+0x49/0xbe
-> #0 (kn->count#202){++++}:
check_prev_add+0x5d2/0xc45
validate_chain+0xed3/0xf94
__lock_acquire+0x95f/0xa2f
lock_acquire+0x1b4/0x1e8
__kernfs_remove+0x237/0x40b
kernfs_remove_by_name_ns+0x59/0x72
remove_files+0x61/0x96
sysfs_remove_group+0x81/0xa4
sysfs_remove_groups+0x3b/0x44
kobject_del+0x44/0x94
blk_mq_unregister_dev+0x83/0xdd
blk_unregister_queue+0xa0/0x10b
del_gendisk+0x259/0x3fa
null_del_dev+0x8b/0x1c3 [null_blk]
null_exit+0x5c/0x95 [null_blk]
__se_sys_delete_module+0x204/0x337
do_syscall_64+0xa7/0x295
entry_SYSCALL_64_after_hwframe+0x49/0xbe
other info that might help us debug this:
Possible unsafe locking scenario:
CPU0 CPU1
---- ----
lock(&q->sysfs_lock);
lock(kn->count#202);
lock(&q->sysfs_lock);
lock(kn->count#202);
*** DEADLOCK ***
2 locks held by rmmod/777:
#0: 00000000e69bd9de (&lock){+.+.}, at: null_exit+0x2e/0x95 [null_blk]
#1: 00000000fb16ae21 (&q->sysfs_lock){+.+.}, at: blk_unregister_queue+0x78/0x10b
stack backtrace:
CPU: 0 PID: 777 Comm: rmmod Not tainted 5.3.0-rc3-00044-g73277fc75ea0 #1380
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS ?-20180724_192412-buildhw-07.phx4
Call Trace:
dump_stack+0x9a/0xe6
check_noncircular+0x207/0x251
? print_circular_bug+0x32a/0x32a
? find_usage_backwards+0x84/0xb0
check_prev_add+0x5d2/0xc45
validate_chain+0xed3/0xf94
? check_prev_add+0xc45/0xc45
? mark_lock+0x11b/0x804
? check_usage_forwards+0x1ca/0x1ca
__lock_acquire+0x95f/0xa2f
lock_acquire+0x1b4/0x1e8
? kernfs_remove_by_name_ns+0x59/0x72
__kernfs_remove+0x237/0x40b
? kernfs_remove_by_name_ns+0x59/0x72
? kernfs_next_descendant_post+0x7d/0x7d
? strlen+0x10/0x23
? strcmp+0x22/0x44
kernfs_remove_by_name_ns+0x59/0x72
remove_files+0x61/0x96
sysfs_remove_group+0x81/0xa4
sysfs_remove_groups+0x3b/0x44
kobject_del+0x44/0x94
blk_mq_unregister_dev+0x83/0xdd
blk_unregister_queue+0xa0/0x10b
del_gendisk+0x259/0x3fa
? disk_events_poll_msecs_store+0x12b/0x12b
? check_flags+0x1ea/0x204
? mark_held_locks+0x1f/0x7a
null_del_dev+0x8b/0x1c3 [null_blk]
null_exit+0x5c/0x95 [null_blk]
__se_sys_delete_module+0x204/0x337
? free_module+0x39f/0x39f
? blkcg_maybe_throttle_current+0x8a/0x718
? rwlock_bug+0x62/0x62
? __blkcg_punt_bio_submit+0xd0/0xd0
? trace_hardirqs_on_thunk+0x1a/0x20
? mark_held_locks+0x1f/0x7a
? do_syscall_64+0x4c/0x295
do_syscall_64+0xa7/0x295
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7fb696cdbe6b
Code: 73 01 c3 48 8b 0d 1d 20 0c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 2e 0f 1f 84 00 00 008
RSP: 002b:00007ffec9588788 EFLAGS: 00000206 ORIG_RAX: 00000000000000b0
RAX: ffffffffffffffda RBX: 0000559e589137c0 RCX: 00007fb696cdbe6b
RDX: 000000000000000a RSI: 0000000000000800 RDI: 0000559e58913828
RBP: 0000000000000000 R08: 00007ffec9587701 R09: 0000000000000000
R10: 00007fb696d4eae0 R11: 0000000000000206 R12: 00007ffec95889b0
R13: 00007ffec95896b3 R14: 0000559e58913260 R15: 0000559e589137c0
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Cc: Hannes Reinecke <hare@xxxxxxxx>
Cc: Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx>
Cc: Mike Snitzer <snitzer@xxxxxxxxxx>
Reviewed-by: Bart Van Assche <bvanassche@xxxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
(jwang:cherry picked from commit cecf5d87ff2035127bb5a9ee054d0023a4a7cad3,
adjust ctx for 4,19)
Signed-off-by: Jack Wang <jinpu.wang@xxxxxxxxxxxxxxx>
---
block/blk-core.c | 1 +
block/blk-mq-sysfs.c | 12 ++++-----
block/blk-sysfs.c | 44 +++++++++++++++++++------------
block/blk.h | 2 +-
block/elevator.c | 59 +++++++++++++++++++++++++++++++++++-------
include/linux/blkdev.h | 1 +
6 files changed, 86 insertions(+), 33 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index ce3710404544..80f3e729fdd4 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1059,6 +1059,7 @@ struct request_queue *blk_alloc_queue_node(gfp_t gfp_mask, int node_id,
mutex_init(&q->blk_trace_mutex);
#endif
mutex_init(&q->sysfs_lock);
+ mutex_init(&q->sysfs_dir_lock);
spin_lock_init(&q->__queue_lock);
if (!q->mq_ops)
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 5006a0d00990..5e4b7ed1e897 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -264,7 +264,7 @@ void blk_mq_unregister_dev(struct device *dev, struct request_queue *q)
struct blk_mq_hw_ctx *hctx;
int i;
- lockdep_assert_held(&q->sysfs_lock);
+ lockdep_assert_held(&q->sysfs_dir_lock);
queue_for_each_hw_ctx(q, hctx, i)
blk_mq_unregister_hctx(hctx);
@@ -312,7 +312,7 @@ int __blk_mq_register_dev(struct device *dev, struct request_queue *q)
int ret, i;
WARN_ON_ONCE(!q->kobj.parent);
- lockdep_assert_held(&q->sysfs_lock);
+ lockdep_assert_held(&q->sysfs_dir_lock);
ret = kobject_add(&q->mq_kobj, kobject_get(&dev->kobj), "%s", "mq");
if (ret < 0)
@@ -358,7 +358,7 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
struct blk_mq_hw_ctx *hctx;
int i;
- mutex_lock(&q->sysfs_lock);
+ mutex_lock(&q->sysfs_dir_lock);
if (!q->mq_sysfs_init_done)
goto unlock;
@@ -366,7 +366,7 @@ void blk_mq_sysfs_unregister(struct request_queue *q)
blk_mq_unregister_hctx(hctx);
unlock:
- mutex_unlock(&q->sysfs_lock);
+ mutex_unlock(&q->sysfs_dir_lock);
}
int blk_mq_sysfs_register(struct request_queue *q)
@@ -374,7 +374,7 @@ int blk_mq_sysfs_register(struct request_queue *q)
struct blk_mq_hw_ctx *hctx;
int i, ret = 0;
- mutex_lock(&q->sysfs_lock);
+ mutex_lock(&q->sysfs_dir_lock);
if (!q->mq_sysfs_init_done)
goto unlock;
@@ -385,7 +385,7 @@ int blk_mq_sysfs_register(struct request_queue *q)
}
unlock:
- mutex_unlock(&q->sysfs_lock);
+ mutex_unlock(&q->sysfs_dir_lock);
return ret;
}
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 0a7636d24563..b2208b69f04a 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -892,6 +892,7 @@ int blk_register_queue(struct gendisk *disk)
int ret;
struct device *dev = disk_to_dev(disk);
struct request_queue *q = disk->queue;
+ bool has_elevator = false;
if (WARN_ON(!q))
return -ENXIO;
@@ -899,7 +900,6 @@ int blk_register_queue(struct gendisk *disk)
WARN_ONCE(blk_queue_registered(q),
"%s is registering an already registered queue\n",
kobject_name(&dev->kobj));
- queue_flag_set_unlocked(QUEUE_FLAG_REGISTERED, q);
/*
* SCSI probing may synchronously create and destroy a lot of
@@ -920,8 +920,7 @@ int blk_register_queue(struct gendisk *disk)
if (ret)
return ret;
- /* Prevent changes through sysfs until registration is completed. */
- mutex_lock(&q->sysfs_lock);
+ mutex_lock(&q->sysfs_dir_lock);
ret = kobject_add(&q->kobj, kobject_get(&dev->kobj), "%s", "queue");
if (ret < 0) {
@@ -934,26 +933,36 @@ int blk_register_queue(struct gendisk *disk)
blk_mq_debugfs_register(q);
}
- kobject_uevent(&q->kobj, KOBJ_ADD);
-
- wbt_enable_default(q);
-
- blk_throtl_register_queue(q);
-
+ /*
+ * The flag of QUEUE_FLAG_REGISTERED isn't set yet, so elevator
+ * switch won't happen at all.
+ */
if (q->request_fn || (q->mq_ops && q->elevator)) {
- ret = elv_register_queue(q);
+ ret = elv_register_queue(q, false);
if (ret) {
- mutex_unlock(&q->sysfs_lock);
- kobject_uevent(&q->kobj, KOBJ_REMOVE);
+ mutex_unlock(&q->sysfs_dir_lock);
kobject_del(&q->kobj);
blk_trace_remove_sysfs(dev);
kobject_put(&dev->kobj);
return ret;
}
+ has_elevator = true;
}
+
+ mutex_lock(&q->sysfs_lock);
+ blk_queue_flag_set(QUEUE_FLAG_REGISTERED, q);
+ wbt_enable_default(q);
+ blk_throtl_register_queue(q);
+
+ /* Now everything is ready and send out KOBJ_ADD uevent */
+ kobject_uevent(&q->kobj, KOBJ_ADD);
+ if (has_elevator)
+ kobject_uevent(&q->elevator->kobj, KOBJ_ADD);
+ mutex_unlock(&q->sysfs_lock);
+
ret = 0;
unlock:
- mutex_unlock(&q->sysfs_lock);
+ mutex_unlock(&q->sysfs_dir_lock);
return ret;
}
EXPORT_SYMBOL_GPL(blk_register_queue);
@@ -968,6 +977,7 @@ EXPORT_SYMBOL_GPL(blk_register_queue);
void blk_unregister_queue(struct gendisk *disk)
{
struct request_queue *q = disk->queue;
+ bool has_elevator;
if (WARN_ON(!q))
return;
@@ -982,25 +992,27 @@ void blk_unregister_queue(struct gendisk *disk)
* concurrent elv_iosched_store() calls.
*/
mutex_lock(&q->sysfs_lock);
-
blk_queue_flag_clear(QUEUE_FLAG_REGISTERED, q);
+ has_elevator = !!q->elevator;
+ mutex_unlock(&q->sysfs_lock);
+ mutex_lock(&q->sysfs_dir_lock);
/*
* Remove the sysfs attributes before unregistering the queue data
* structures that can be modified through sysfs.
*/
if (q->mq_ops)
blk_mq_unregister_dev(disk_to_dev(disk), q);
- mutex_unlock(&q->sysfs_lock);
kobject_uevent(&q->kobj, KOBJ_REMOVE);
kobject_del(&q->kobj);
blk_trace_remove_sysfs(disk_to_dev(disk));
mutex_lock(&q->sysfs_lock);
- if (q->request_fn || (q->mq_ops && q->elevator))
+ if (q->request_fn || has_elevator)
elv_unregister_queue(q);
mutex_unlock(&q->sysfs_lock);
+ mutex_unlock(&q->sysfs_dir_lock);
kobject_put(&disk_to_dev(disk)->kobj);
}
diff --git a/block/blk.h b/block/blk.h
index 1a5b67b57e6b..ae87e2a5f2bd 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -244,7 +244,7 @@ int elevator_init_mq(struct request_queue *q);
int elevator_switch_mq(struct request_queue *q,
struct elevator_type *new_e);
void elevator_exit(struct request_queue *, struct elevator_queue *);
-int elv_register_queue(struct request_queue *q);
+int elv_register_queue(struct request_queue *q, bool uevent);
void elv_unregister_queue(struct request_queue *q);
struct hd_struct *__disk_get_part(struct gendisk *disk, int partno);
diff --git a/block/elevator.c b/block/elevator.c
index 9bffe4558929..2ff0859e8b35 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -833,13 +833,16 @@ static struct kobj_type elv_ktype = {
.release = elevator_release,
};
-int elv_register_queue(struct request_queue *q)
+/*
+ * elv_register_queue is called from either blk_register_queue or
+ * elevator_switch, elevator switch is prevented from being happen
+ * in the two paths, so it is safe to not hold q->sysfs_lock.
+ */
+int elv_register_queue(struct request_queue *q, bool uevent)
{
struct elevator_queue *e = q->elevator;
int error;
- lockdep_assert_held(&q->sysfs_lock);
-
error = kobject_add(&e->kobj, &q->kobj, "%s", "iosched");
if (!error) {
struct elv_fs_entry *attr = e->type->elevator_attrs;
@@ -850,26 +853,36 @@ int elv_register_queue(struct request_queue *q)
attr++;
}
}
- kobject_uevent(&e->kobj, KOBJ_ADD);
+ if (uevent)
+ kobject_uevent(&e->kobj, KOBJ_ADD);
+
+ mutex_lock(&q->sysfs_lock);
e->registered = 1;
if (!e->uses_mq && e->type->ops.sq.elevator_registered_fn)
e->type->ops.sq.elevator_registered_fn(q);
+ mutex_unlock(&q->sysfs_lock);
}
return error;
}
+/*
+ * elv_unregister_queue is called from either blk_unregister_queue or
+ * elevator_switch, elevator switch is prevented from being happen
+ * in the two paths, so it is safe to not hold q->sysfs_lock.
+ */
void elv_unregister_queue(struct request_queue *q)
{
- lockdep_assert_held(&q->sysfs_lock);
-
if (q) {
struct elevator_queue *e = q->elevator;
kobject_uevent(&e->kobj, KOBJ_REMOVE);
kobject_del(&e->kobj);
+
+ mutex_lock(&q->sysfs_lock);
e->registered = 0;
/* Re-enable throttling in case elevator disabled it */
wbt_enable_default(q);
+ mutex_unlock(&q->sysfs_lock);
}
}
@@ -940,10 +953,32 @@ int elevator_switch_mq(struct request_queue *q,
lockdep_assert_held(&q->sysfs_lock);
if (q->elevator) {
- if (q->elevator->registered)
+ if (q->elevator->registered) {
+ mutex_unlock(&q->sysfs_lock);
+
+ /*
+ * Concurrent elevator switch can't happen becasue
+ * sysfs write is always exclusively on same file.
+ *
+ * Also the elevator queue won't be freed after
+ * sysfs_lock is released becasue kobject_del() in
+ * blk_unregister_queue() waits for completion of
+ * .store & .show on its attributes.
+ */
elv_unregister_queue(q);
+
+ mutex_lock(&q->sysfs_lock);
+ }
ioc_clear_queue(q);
elevator_exit(q, q->elevator);
+
+ /*
+ * sysfs_lock may be dropped, so re-check if queue is
+ * unregistered. If yes, don't switch to new elevator
+ * any more
+ */
+ if (!blk_queue_registered(q))
+ return 0;
}
ret = blk_mq_init_sched(q, new_e);
@@ -951,7 +986,11 @@ int elevator_switch_mq(struct request_queue *q,
goto out;
if (new_e) {
- ret = elv_register_queue(q);
+ mutex_unlock(&q->sysfs_lock);
+
+ ret = elv_register_queue(q, true);
+
+ mutex_lock(&q->sysfs_lock);
if (ret) {
elevator_exit(q, q->elevator);
goto out;
@@ -1047,7 +1086,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
if (err)
goto fail_init;
- err = elv_register_queue(q);
+ err = elv_register_queue(q, true);
if (err)
goto fail_register;
@@ -1067,7 +1106,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
/* switch failed, restore and re-register old elevator */
if (old) {
q->elevator = old;
- elv_register_queue(q);
+ elv_register_queue(q, true);
blk_queue_bypass_end(q);
}
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 3a2b34c2c82b..209ba8e7bd31 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -637,6 +637,7 @@ struct request_queue {
struct delayed_work requeue_work;
struct mutex sysfs_lock;
+ struct mutex sysfs_dir_lock;
int bypass_depth;
atomic_t mq_freeze_depth;
--
2.25.1
