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Date:   Wed, 13 Oct 2021 20:44:56 +0800
From:   Yu Kuai <yukuai3@...wei.com>
To:     <tj@...nel.org>, <axboe@...nel.dk>, <paolo.valente@...aro.org>,
        <fchecconi@...il.com>, <avanzini.arianna@...il.com>,
        <mkoutny@...e.com>
CC:     <cgroups@...r.kernel.org>, <linux-block@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <yukuai3@...wei.com>,
        <yi.zhang@...wei.com>
Subject: [PATCH v2 -next] blk-cgroup: synchoronize blkg creation against policy deactivation

Out test report a null pointer dereference:

[  168.534653] ==================================================================
[  168.535614] Disabling lock debugging due to kernel taint
[  168.536346] BUG: kernel NULL pointer dereference, address: 0000000000000008
[  168.537274] #PF: supervisor read access in kernel mode
[  168.537964] #PF: error_code(0x0000) - not-present page
[  168.538667] PGD 0 P4D 0
[  168.539025] Oops: 0000 [#1] PREEMPT SMP KASAN
[  168.539656] CPU: 13 PID: 759 Comm: bash Tainted: G    B             5.15.0-rc2-next-202100
[  168.540954] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS ?-20190727_0738364
[  168.542736] RIP: 0010:bfq_pd_init+0x88/0x1e0
[  168.543318] Code: 98 00 00 00 e8 c9 e4 5b ff 4c 8b 65 00 49 8d 7c 24 08 e8 bb e4 5b ff 4d0
[  168.545803] RSP: 0018:ffff88817095f9c0 EFLAGS: 00010002
[  168.546497] RAX: 0000000000000001 RBX: ffff888101a1c000 RCX: 0000000000000000
[  168.547438] RDX: 0000000000000003 RSI: 0000000000000002 RDI: ffff888106553428
[  168.548402] RBP: ffff888106553400 R08: ffffffff961bcaf4 R09: 0000000000000001
[  168.549365] R10: ffffffffa2e16c27 R11: fffffbfff45c2d84 R12: 0000000000000000
[  168.550291] R13: ffff888101a1c098 R14: ffff88810c7a08c8 R15: ffffffffa55541a0
[  168.551221] FS:  00007fac75227700(0000) GS:ffff88839ba80000(0000) knlGS:0000000000000000
[  168.552278] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  168.553040] CR2: 0000000000000008 CR3: 0000000165ce7000 CR4: 00000000000006e0
[  168.554000] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  168.554929] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  168.555888] Call Trace:
[  168.556221]  <TASK>
[  168.556510]  blkg_create+0x1c0/0x8c0
[  168.556989]  blkg_conf_prep+0x574/0x650
[  168.557502]  ? stack_trace_save+0x99/0xd0
[  168.558033]  ? blkcg_conf_open_bdev+0x1b0/0x1b0
[  168.558629]  tg_set_conf.constprop.0+0xb9/0x280
[  168.559231]  ? kasan_set_track+0x29/0x40
[  168.559758]  ? kasan_set_free_info+0x30/0x60
[  168.560344]  ? tg_set_limit+0xae0/0xae0
[  168.560853]  ? do_sys_openat2+0x33b/0x640
[  168.561383]  ? do_sys_open+0xa2/0x100
[  168.561877]  ? __x64_sys_open+0x4e/0x60
[  168.562383]  ? __kasan_check_write+0x20/0x30
[  168.562951]  ? copyin+0x48/0x70
[  168.563390]  ? _copy_from_iter+0x234/0x9e0
[  168.563948]  tg_set_conf_u64+0x17/0x20
[  168.564467]  cgroup_file_write+0x1ad/0x380
[  168.565014]  ? cgroup_file_poll+0x80/0x80
[  168.565568]  ? __mutex_lock_slowpath+0x30/0x30
[  168.566165]  ? pgd_free+0x100/0x160
[  168.566649]  kernfs_fop_write_iter+0x21d/0x340
[  168.567246]  ? cgroup_file_poll+0x80/0x80
[  168.567796]  new_sync_write+0x29f/0x3c0
[  168.568314]  ? new_sync_read+0x410/0x410
[  168.568840]  ? __handle_mm_fault+0x1c97/0x2d80
[  168.569425]  ? copy_page_range+0x2b10/0x2b10
[  168.570007]  ? _raw_read_lock_bh+0xa0/0xa0
[  168.570622]  vfs_write+0x46e/0x630
[  168.571091]  ksys_write+0xcd/0x1e0
[  168.571563]  ? __x64_sys_read+0x60/0x60
[  168.572081]  ? __kasan_check_write+0x20/0x30
[  168.572659]  ? do_user_addr_fault+0x446/0xff0
[  168.573264]  __x64_sys_write+0x46/0x60
[  168.573774]  do_syscall_64+0x35/0x80
[  168.574264]  entry_SYSCALL_64_after_hwframe+0x44/0xae
[  168.574960] RIP: 0033:0x7fac74915130
[  168.575456] Code: 73 01 c3 48 8b 0d 58 ed 2c 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 444
[  168.577969] RSP: 002b:00007ffc3080e288 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[  168.578986] RAX: ffffffffffffffda RBX: 0000000000000009 RCX: 00007fac74915130
[  168.579937] RDX: 0000000000000009 RSI: 000056007669f080 RDI: 0000000000000001
[  168.580884] RBP: 000056007669f080 R08: 000000000000000a R09: 00007fac75227700
[  168.581841] R10: 000056007655c8f0 R11: 0000000000000246 R12: 0000000000000009
[  168.582796] R13: 0000000000000001 R14: 00007fac74be55e0 R15: 00007fac74be08c0
[  168.583757]  </TASK>
[  168.584063] Modules linked in:
[  168.584494] CR2: 0000000000000008
[  168.584964] ---[ end trace 2475611ad0f77a1a ]---

This is because blkg_alloc() is called from blkg_conf_prep() without
holding 'q->queue_lock', and elevator is exited before blkg_create():

thread 1                            thread 2
blkg_conf_prep
 spin_lock_irq(&q->queue_lock);
 blkg_lookup_check -> return NULL
 spin_unlock_irq(&q->queue_lock);

 blkg_alloc
  blkcg_policy_enabled -> true
  pd = ->pd_alloc_fn
  blkg->pd[i] = pd
                                   blk_mq_exit_sched
                                    bfq_exit_queue
                                     blkcg_deactivate_policy
                                      spin_lock_irq(&q->queue_lock);
                                      __clear_bit(pol->plid, q->blkcg_pols);
                                      spin_unlock_irq(&q->queue_lock);
                                    q->elevator = NULL;
  spin_lock_irq(&q->queue_lock);
   blkg_create
    if (blkg->pd[i])
     ->pd_init_fn -> q->elevator is NULL
  spin_unlock_irq(&q->queue_lock);

Because blkcg_deactivate_policy() can take a long time if there are too
many blkgs in q->blkg_list, fix the problem by adding a mutex in q
and using it to synchoronize blkg creation against policy deactivation.

Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
Signed-off-by: Yu Kuai <yukuai3@...wei.com>
---
Changes in V2:
 - rename the patch title
 - instead of checking policy in blkg_create(), using a new solution.

 block/blk-cgroup.c     | 7 +++++++
 block/blk-core.c       | 1 +
 include/linux/blkdev.h | 5 +++++
 3 files changed, 13 insertions(+)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index eb48090eefce..043f85a1e05e 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -621,6 +621,7 @@ struct block_device *blkcg_conf_open_bdev(char **inputp)
  */
 int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 		   char *input, struct blkg_conf_ctx *ctx)
+	__acquires(&bdev->bd_disk->queue->blkg_lock)
 	__acquires(rcu) __acquires(&bdev->bd_disk->queue->queue_lock)
 {
 	struct block_device *bdev;
@@ -634,6 +635,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 
 	q = bdev->bd_disk->queue;
 
+	mutex_lock(&q->blkg_lock);
 	rcu_read_lock();
 	spin_lock_irq(&q->queue_lock);
 
@@ -713,6 +715,7 @@ int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
 fail_unlock:
 	spin_unlock_irq(&q->queue_lock);
 	rcu_read_unlock();
+	mutex_unlock(&q->blkg_lock);
 fail:
 	blkdev_put_no_open(bdev);
 	/*
@@ -738,9 +741,11 @@ EXPORT_SYMBOL_GPL(blkg_conf_prep);
  */
 void blkg_conf_finish(struct blkg_conf_ctx *ctx)
 	__releases(&ctx->bdev->bd_disk->queue->queue_lock) __releases(rcu)
+	__releases(&ctx->bdev->bd_disk->queue->blkg_lock)
 {
 	spin_unlock_irq(&ctx->bdev->bd_disk->queue->queue_lock);
 	rcu_read_unlock();
+	mutex_unlock(&ctx->bdev->bd_disk->queue->blkg_lock);
 	blkdev_put_no_open(ctx->bdev);
 }
 EXPORT_SYMBOL_GPL(blkg_conf_finish);
@@ -1401,6 +1406,7 @@ void blkcg_deactivate_policy(struct request_queue *q,
 	if (queue_is_mq(q))
 		blk_mq_freeze_queue(q);
 
+	mutex_lock(&q->blkg_lock);
 	spin_lock_irq(&q->queue_lock);
 
 	__clear_bit(pol->plid, q->blkcg_pols);
@@ -1419,6 +1425,7 @@ void blkcg_deactivate_policy(struct request_queue *q,
 	}
 
 	spin_unlock_irq(&q->queue_lock);
+	mutex_unlock(&q->blkg_lock);
 
 	if (queue_is_mq(q))
 		blk_mq_unfreeze_queue(q);
diff --git a/block/blk-core.c b/block/blk-core.c
index d83e56b2f64e..f37fc03b1113 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -544,6 +544,7 @@ struct request_queue *blk_alloc_queue(int node_id)
 	INIT_LIST_HEAD(&q->icq_list);
 #ifdef CONFIG_BLK_CGROUP
 	INIT_LIST_HEAD(&q->blkg_list);
+	mutex_init(&q->blkg_lock);
 #endif
 
 	kobject_init(&q->kobj, &blk_queue_ktype);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index b19172db7eef..c2d5be0f5f37 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -245,6 +245,11 @@ struct request_queue {
 	DECLARE_BITMAP		(blkcg_pols, BLKCG_MAX_POLS);
 	struct blkcg_gq		*root_blkg;
 	struct list_head	blkg_list;
+	/*
+	 * used to synchronize blkg allocation and initialization against
+	 * policy deactivation.
+	 */
+	struct mutex		blkg_lock;
 #endif
 
 	struct queue_limits	limits;
-- 
2.31.1

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