[PATCH v2] ALSA: seq: Fix RCU stall in snd_seq_write()

[Zqiang]

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 at syzkaller.appspotmail.com
Signed-off-by: Zqiang <qiang.zhang1211 at gmail.com>
---
 v1->v2:
 Add atomic context detection.

 sound/core/seq/seq_queue.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/sound/core/seq/seq_queue.c b/sound/core/seq/seq_queue.c
index d6c02dea976c..3a05b5e0d555 100644
--- a/sound/core/seq/seq_queue.c
+++ b/sound/core/seq/seq_queue.c
@@ -263,6 +263,8 @@ 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 (!atomic)
+			cond_resched();
 	}
 
 	/* Process time queue... */
@@ -272,6 +274,8 @@ 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 (!atomic)
+			cond_resched();
 	}
 
 	/* free lock */
-- 
2.17.1