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Subject: Re: [PATCH] ALSA: seq: Fix RCU stall in snd_seq_write()
To:     Takashi Iwai <tiwai@suse.de>
Cc:     tiwai@suse.com, alsa-devel@alsa-project.org,
        linux-kernel@vger.kernel.org
References: <20211102033222.3849-1-qiang.zhang1211@gmail.com>
 <s5hy267ot27.wl-tiwai@suse.de>
 <2d05ceab-b8b7-0c7b-f847-69950c6db14e@gmail.com>
 <s5ho872q25p.wl-tiwai@suse.de>
From:   Zqiang <qiang.zhang1211@gmail.com>
Message-ID: <47f05b3a-811b-e64c-0366-3aebaece6c8e@gmail.com>
Date:   Tue, 2 Nov 2021 19:20:32 +0800
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On 2021/11/2 下午6:31, Takashi Iwai wrote:
> On Tue, 02 Nov 2021 10:41:57 +0100,
> Zqiang wrote:
>>
>> On 2021/11/2 下午4:33, Takashi Iwai wrote:
>>> On Tue, 02 Nov 2021 04:32:22 +0100,
>>> Zqiang wrote:
>>>> If we have a lot of cell object, this cycle may take a long time, and
>>>> trigger RCU stall. insert a conditional reschedule point to fix it.
>>>>
>>>> rcu: INFO: rcu_preempt self-detected stall on CPU
>>>> rcu: 	1-....: (1 GPs behind) idle=9f5/1/0x4000000000000000
>>>> 	softirq=16474/16475 fqs=4916
>>>> 	(t=10500 jiffies g=19249 q=192515)
>>>> NMI backtrace for cpu 1
>>>> ......
>>>> asm_sysvec_apic_timer_interrupt
>>>> RIP: 0010:_raw_spin_unlock_irqrestore+0x38/0x70
>>>> spin_unlock_irqrestore
>>>> snd_seq_prioq_cell_out+0x1dc/0x360
>>>> snd_seq_check_queue+0x1a6/0x3f0
>>>> snd_seq_enqueue_event+0x1ed/0x3e0
>>>> snd_seq_client_enqueue_event.constprop.0+0x19a/0x3c0
>>>> snd_seq_write+0x2db/0x510
>>>> vfs_write+0x1c4/0x900
>>>> ksys_write+0x171/0x1d0
>>>> do_syscall_64+0x35/0xb0
>>>>
>>>> Reported-by: syzbot+bb950e68b400ab4f65f8@syzkaller.appspotmail.com
>>>> Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
>>>> ---
>>>>    sound/core/seq/seq_queue.c | 2 ++
>>>>    1 file changed, 2 insertions(+)
>>>>
>>>> diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
>>>> index d6c02dea976c..f5b1e4562a64 100644
>>>> --- a/sound/core/seq/seq_queue.c
>>>> +++ b/sound/core/seq/seq_queue.c
>>>> @@ -263,6 +263,7 @@ void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
>>>>    		if (!cell)
>>>>    			break;
>>>>    		snd_seq_dispatch_event(cell, atomic, hop);
>>>> +		cond_resched();
>>>>    	}
>>>>      	/* Process time queue... */
>>>> @@ -272,6 +273,7 @@ void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
>>>>    		if (!cell)
>>>>    			break;
>>>>    		snd_seq_dispatch_event(cell, atomic, hop);
>>>> +		cond_resched();
>>> It's good to have cond_resched() in those places but it must be done
>>> more carefully, as the code path may be called from the non-atomic
>>> context, too.  That is, it must have a check of atomic argument, and
>>> cond_resched() is applied only when atomic==false.
>>>
>>> But I still wonder how this gets a RCU stall out of sudden.  Looking
>>> through https://syzkaller.appspot.com/bug?extid=bb950e68b400ab4f65f8
>>> it's triggered by many cases since the end of September...
>> I did not find useful information from the log,  through calltrace, I
>> guess it may be triggered by the long cycle time, which caused the
>> static state of the RCU to
>>
>> not be reported in time.
> Yes, I understand that logic.  But I wonder why this gets triggered
> *now* out of sudden.  The code has been present over decades, and I
> don't think the similar test case must have been performed by fuzzer.
>
>> I ignore the atomic parameter check,  I will resend v2 .   in
>> no-atomic context, we can insert
>>
>> cond_resched() to avoid this situation, but in atomic context,
>>
>> the RCU stall maybe still trigger.
> Right, so maybe it's better to have an upper limit for the processed
> cells, something like below (totally untested).
>
> Could you reproduce the problem locally?  Otherwise it's all nothing
> but a guess...

yes, this is just a guess.  I haven't reproduced locally, limiting the 
number of cycles is a suitable modification,

but the  MAX_CELL_PROCESSES_IN_QUEUE is an experience value.

thanks

Zqiang

>
> thanks,
>
> Takashi
>
> ---
>
> diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
> index d6c02dea976c..7f796ee62ee7 100644
> --- a/sound/core/seq/seq_queue.c
> +++ b/sound/core/seq/seq_queue.c
> @@ -235,12 +235,15 @@ struct snd_seq_queue *snd_seq_queue_find_name(char *name)
>   
>   /* -------------------------------------------------------- */
>   
> +#define MAX_CELL_PROCESSES_IN_QUEUE	1000
> +
>   void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
>   {
>   	unsigned long flags;
>   	struct snd_seq_event_cell *cell;
>   	snd_seq_tick_time_t cur_tick;
>   	snd_seq_real_time_t cur_time;
> +	int processed = 0;
>   
>   	if (q == NULL)
>   		return;
> @@ -263,6 +266,8 @@ void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
>   		if (!cell)
>   			break;
>   		snd_seq_dispatch_event(cell, atomic, hop);
> +		if (++processed >= MAX_CELL_PROCESSES_IN_QUEUE)
> +			return; /* the rest processed at the next batch */
>   	}
>   
>   	/* Process time queue... */
> @@ -272,6 +277,8 @@ void snd_seq_check_queue(struct snd_seq_queue *q, int atomic, int hop)
>   		if (!cell)
>   			break;
>   		snd_seq_dispatch_event(cell, atomic, hop);
> +		if (++processed >= MAX_CELL_PROCESSES_IN_QUEUE)
> +			return; /* the rest processed at the next batch */
>   	}
>   
>   	/* free lock */