Re: [PATCH] ALSA: seq: Fix RCU stall in snd_seq_write()

[Zqiang]

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@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Signed-off-by: Zqiang <qiang.zhang1211@xxxxxxxxx>
> > ---
> >   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.

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.

thanks
Zqiang

> thanks,
>
> Takashi