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
