On 2021/11/2 下午8:27, Takashi Iwai wrote:
On Tue, 02 Nov 2021 12:20:32 +0100, Zqiang wrote:
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.
Yes, that's why we need the reproducer in anyway before moving forward. The problem is that the patch looks as if it were fixing the RCU stall, but we haven't verified it at all that it is really the cause. Even we haven't checked whether it's really the too many cells queued, or just because the concurrent queuing made the function re-running.
Thanks your explanation, I think we can send your changes out and wait for syzbot to test.
thanks
Zqiang
Takashi