linux-kernel - Re: [PATCH] blk-mq: don't queue 'hctx->run

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Date:   Sat, 14 May 2022 17:29:40 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     <axboe@...nel.dk>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH] blk-mq: don't queue 'hctx->run_work' if the queue is dead

friendly ping ...

在 2022/05/10 19:23, Yu Kuai 写道:
> Our test report a following crash:
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000018
> PGD 0 P4D 0
> Oops: 0000 [#1] SMP NOPTI
> CPU: 6 PID: 265 Comm: kworker/6:1H Kdump: loaded Tainted: G           O      5.10.0-60.17.0.h43.eulerosv2r11.x86_64 #1
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.1-0-ga5cab58-20220320_160524-szxrtosci10000 04/01/2014
> Workqueue: kblockd blk_mq_run_work_fn
> RIP: 0010:blk_mq_delay_run_hw_queues+0xb6/0xe0
> RSP: 0018:ffffacc6803d3d88 EFLAGS: 00010246
> RAX: 0000000000000006 RBX: ffff99e2c3d25008 RCX: 00000000ffffffff
> RDX: 0000000000000000 RSI: 0000000000000003 RDI: ffff99e2c911ae18
> RBP: ffffacc6803d3dd8 R08: 0000000000000000 R09: ffff99e2c0901f6c
> R10: 0000000000000018 R11: 0000000000000018 R12: ffff99e2c911ae18
> R13: 0000000000000000 R14: 0000000000000003 R15: ffff99e2c911ae18
> FS:  0000000000000000(0000) GS:ffff99e6bbf00000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000018 CR3: 000000007460a006 CR4: 00000000003706e0
> Call Trace:
>   __blk_mq_do_dispatch_sched+0x2a7/0x2c0
>   ? newidle_balance+0x23e/0x2f0
>   __blk_mq_sched_dispatch_requests+0x13f/0x190
>   blk_mq_sched_dispatch_requests+0x30/0x60
>   __blk_mq_run_hw_queue+0x47/0xd0
>   process_one_work+0x1b0/0x350
>   worker_thread+0x49/0x300
>   ? rescuer_thread+0x3a0/0x3a0
>   kthread+0xfe/0x140
>   ? kthread_park+0x90/0x90
>   ret_from_fork+0x22/0x30
> 
> After digging from vmcore, I found that the queue is cleaned
> up(blk_cleanup_queue() is done) and tag set is
> freed(blk_mq_free_tag_set() is done).
> 
> There are two problems here:
> 
> 1) blk_mq_delay_run_hw_queues() will only be called from
> __blk_mq_do_dispatch_sched() if e->type->ops.has_work() return true.
> This seems impossible because blk_cleanup_queue() is done, and there
> should be no io. However, bfq_has_work() can return true even if no
> io is queued. This is because bfq_has_work() is using busy queues, and
> bfq_queue can stay busy after dispatching all the requests.
> 
> 2) 'hctx->run_work' still exists after blk_cleanup_queue().
> blk_mq_cancel_work_sync() is called from blk_cleanup_queue() to cancel
> all the 'run_work'. However, there is no guarantee that new 'run_work'
> won't be queued after that(and before blk_mq_exit_queue() is done).
> 
> The first problem is not the root cause, this patch just fix the second
> problem by checking the 'QUEUE_FLAG_DEAD' before queuing 'hctx->run_work',
> and using 'queue_lock' to synchronize queuing new work and cacelling the
> old work.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>   block/blk-core.c |  3 +++
>   block/blk-mq.c   | 10 ++++++++--
>   2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index c3f1e46ddd43..2609805861be 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -318,7 +318,10 @@ void blk_cleanup_queue(struct request_queue *q)
>   	/* cleanup rq qos structures for queue without disk */
>   	rq_qos_exit(q);
>   
> +	/* New 'hctx->run_work' can't be queued after setting the dead flag */
> +	spin_lock_irq(&q->queue_lock);
>   	blk_queue_flag_set(QUEUE_FLAG_DEAD, q);
> +	spin_unlock_irq(&q->queue_lock);
>   
>   	blk_sync_queue(q);
>   	if (queue_is_mq(q)) {
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 2cf011b57cf9..34b4914204dd 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2042,6 +2042,8 @@ static int blk_mq_hctx_next_cpu(struct blk_mq_hw_ctx *hctx)
>   static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
>   					unsigned long msecs)
>   {
> +	unsigned long flag;
> +
>   	if (unlikely(blk_mq_hctx_stopped(hctx)))
>   		return;
>   
> @@ -2056,8 +2058,12 @@ static void __blk_mq_delay_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async,
>   		put_cpu();
>   	}
>   
> -	kblockd_mod_delayed_work_on(blk_mq_hctx_next_cpu(hctx), &hctx->run_work,
> -				    msecs_to_jiffies(msecs));
> +	spin_lock_irqsave(&hctx->queue->queue_lock, flag);
> +	if (!blk_queue_dead(hctx->queue))
> +		kblockd_mod_delayed_work_on(blk_mq_hctx_next_cpu(hctx),
> +					    &hctx->run_work,
> +					    msecs_to_jiffies(msecs));
> +	spin_unlock_irqrestore(&hctx->queue->queue_lock, flag);
>   }
>   
>   /**
>