[PATCH] blk-cgroup: fix circular locking dependency in blkg_conf_prep

[Sudarsan Mahendran]

In blkg_conf_prep, we may do IO when allocating while
holding blkcg_mutex, however this presents a deadlock
scenario since we may grab sysfs_lock while IO is
suspended in elevator_exit, and an existing chain
exists from sysfs_mutex -> blkcg_mutex.

We can fix this by performing only GFP_NOIO allocations within
blkg_conf_prep when blkcg_mutex is held.
This patch fixes our reproducer.

Full lockdep splat:

[ 3795.003422] ======================================================
[ 3795.009595] WARNING: possible circular locking dependency detected
[ 3795.015773] 6.18.20-dbg-DEV #1 Tainted: G                 N
[ 3795.021432] ------------------------------------------------------
[ 3795.027604] kpython3/435194 is trying to acquire lock:
[ 3795.032744] ffffffffa36959b8 (fs_reclaim){+.+.}-{0:0}, at: kmem_cache_alloc_noprof+0x49/0x5c0
[ 3795.041260]
[ 3795.041260] but task is already holding lock:
[ 3795.047086] ffff98c8a06477b0 (&q->blkcg_mutex){+.+.}-{4:4}, at: blkg_conf_prep+0x71/0x5e0
[ 3795.055257]
[ 3795.055257] which lock already depends on the new lock.
[ 3795.055257]
[ 3795.063422]
[ 3795.063422] the existing dependency chain (in reverse order) is:
[ 3795.070893]
[ 3795.070893] -> #4 (&q->blkcg_mutex){+.+.}-{4:4}:
[ 3795.076986]        __mutex_lock+0x92/0xb70
[ 3795.081085]        blkcg_deactivate_policy+0x7f/0x230
[ 3795.086137]        bfq_exit_queue+0xe1/0x110
[ 3795.090401]        blk_mq_exit_sched+0xd1/0x170
[ 3795.094935]        elevator_exit+0x50/0x70
(...)
[ 3795.135019]
[ 3795.135019] -> #3 (&eq->sysfs_lock){+.+.}-{4:4}:
[ 3795.141110]        __mutex_lock+0x92/0xb70
[ 3795.145211]        elevator_exit+0x45/0x70
[ 3795.149308]        elevator_switch+0x98/0x290
[ 3795.153660]        elevator_change+0x109/0x190
[ 3795.158106]        elv_iosched_store+0x198/0x240
(...)
[ 3795.185276]
[ 3795.185276] -> #2 (&q->elevator_lock){+.+.}-{4:4}:
[ 3795.191541]        __mutex_lock+0x92/0xb70
[ 3795.195642]        elevator_change+0xb6/0x190
[ 3795.200001]        elv_iosched_store+0x198/0x240
(...)
[ 3795.227179]
[ 3795.227179] -> #1 (&q->q_usage_counter(io)#19){++++}-{0:0}:
[ 3795.234224]        blk_alloc_queue+0x33e/0x370
(...)
[ 3795.266621]
[ 3795.266621] -> #0 (fs_reclaim){+.+.}-{0:0}:
[ 3795.272280]        __lock_acquire+0x157c/0x2980
[ 3795.276813]        lock_acquire+0xd3/0x2a0
[ 3795.280904]        fs_reclaim_acquire+0x55/0xc0
[ 3795.285435]        kmem_cache_alloc_noprof+0x49/0x5c0
[ 3795.290481]        __radix_tree_preload+0xef/0x250
[ 3795.295273]        blkg_conf_prep+0x2aa/0x5e0
[ 3795.299633]        tg_set_conf+0xa5/0x1a0
(...)
[ 3795.330808]
[ 3795.330808] other info that might help us debug this:
[ 3795.330808]
[ 3795.338806] Chain exists of:
[ 3795.338806]   fs_reclaim --> &eq->sysfs_lock --> &q->blkcg_mutex
[ 3795.338806]
[ 3795.349146]  Possible unsafe locking scenario:
[ 3795.349146]
[ 3795.355057]        CPU0                    CPU1
[ 3795.359590]        ----                    ----
[ 3795.364121]   lock(&q->blkcg_mutex);
[ 3795.367691]                                lock(&eq->sysfs_lock);
[ 3795.373776]                                lock(&q->blkcg_mutex);
[ 3795.379862]   lock(fs_reclaim);
[ 3795.383006]
[ 3795.383006]  *** DEADLOCK ***

Reviewed-by: Khazhy Kumykov <khazhy@xxxxxxxxxx>
Signed-off-by: Sudarsan Mahendran <sudarsanm@xxxxxxxxxx>
---
 block/blk-cgroup.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d9676126c5b5..b981a80bbd94 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -867,7 +867,6 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			parent = blkcg_parent(parent);
 		}
 
-		/* Drop locks to do new blkg allocation with GFP_KERNEL. */
 		spin_unlock_irq(&q->queue_lock);
 
 		new_blkg = blkg_alloc(pos, disk, GFP_NOIO);
@@ -876,7 +875,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 			goto fail_exit;
 		}
 
-		if (radix_tree_preload(GFP_KERNEL)) {
+		if (radix_tree_preload(GFP_NOIO)) {
 			blkg_free(new_blkg);
 			ret = -ENOMEM;
 			goto fail_exit;
-- 
2.55.0.679.g6767b8d81c-goog