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Subject: Re: [PATCH v4] blk-cgroup: synchoronize blkg creation against policy
 deactivation
To:     <tj@kernel.org>, <axboe@kernel.dk>, <paolo.valente@linaro.org>,
        <avanzini.arianna@gmail.com>, <fchecconi@gmail.com>
CC:     <cgroups@vger.kernel.org>, <linux-block@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>, <yi.zhang@huawei.com>
References: <20211020014036.2141723-1-yukuai3@huawei.com>
From:   "yukuai (C)" <yukuai3@huawei.com>
Message-ID: <461fa6c1-fbc3-2c66-ed11-8d035c45975a@huawei.com>
Date:   Fri, 22 Oct 2021 09:28:32 +0800
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Hi 2021/10/20 9:40, Yu Kuai wrote:
> 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() requires queue to be frozen, we can
> grab q_usage_counter to synchoronize blkg_conf_prep() against
> blkcg_deactivate_policy().
> 
> Fixes: e21b7a0b9887 ("block, bfq: add full hierarchical scheduling and cgroups support")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> Acked-by: Tejun Heo <tj@kernel.org>

Hi, jens

Can you please apply this patch?

Thanks,
Kuai