Re: get/put_cpu() usage in block/blk-mq.c

[Jens Axboe <axboe@fb.com>]

On 05/09/2017 09:13 AM, Jens Axboe wrote:
> On 05/09/2017 09:04 AM, Mike Galbraith wrote:
>> On Tue, 2017-05-09 at 08:53 -0600, Jens Axboe wrote:
>>> On 05/09/2017 12:07 AM, Mike Galbraith wrote:
>>>> Hi Jens,
>>>>
>>>> I was about to fix up this splat..
>>>>
>>>> [  445.022141] loop: module loaded
>>>> [  445.078116] BUG: using smp_processor_id() in preemptible [00000000] code: loop0/3801
>>>> [  445.085873] caller is debug_smp_processor_id+0x17/0x20
>>>> [  445.091016] CPU: 7 PID: 3801 Comm: loop0 Tainted: G            E   4.12.0-default #40
>>>> [  445.098838] Hardware name: IBM System x3550 M3 -[7944K3G]-/69Y5698     , BIOS -[D6E150AUS-1.10]- 12/15/2010
>>>> [  445.108564] Call Trace:
>>>> [  445.111016]  dump_stack+0x65/0x89
>>>> [  445.114330]  check_preemption_disabled+0xde/0xf0
>>>> [  445.118945]  debug_smp_processor_id+0x17/0x20
>>>> [  445.123300]  blk_stat_add+0xb0/0x130
>>>> [  445.126876]  __blk_mq_complete_request+0xb5/0x150
>>>> [  445.131575]  blk_mq_complete_request+0x16/0x20
>>>> [  445.136020]  loop_queue_work+0x5f/0xaa0 [loop]
>>>> [  445.140461]  ? _raw_spin_unlock_irq+0x21/0x40
>>>> [  445.144816]  ? finish_task_switch+0x85/0x270
>>>> [  445.149085]  ? __schedule+0x291/0x8c0
>>>> [  445.152747]  kthread_worker_fn+0xc2/0x1d0
>>>> [  445.156754]  kthread+0x114/0x150
>>>> [  445.159983]  ? __kthread_init_worker+0xb0/0xb0
>>>> [  445.164424]  ? kthread_park+0x60/0x60
>>>> [  445.168085]  ret_from_fork+0x2c/0x40
>>>
>>> Is it from this_cpu_ptr() in blk_stat_add()?
>>
>> Yeah.
> 
> So why is this complaining, doesn't rcu_read_lock() disable
> preemption?

Ah, I guess it doesn't if PREEMPT_RCU is set. How about the below?


diff --git a/block/blk-stat.c b/block/blk-stat.c
index 6c2f40940439..022f5925a427 100644
--- a/block/blk-stat.c
+++ b/block/blk-stat.c
@@ -96,13 +96,16 @@ void blk_stat_add(struct request *rq)
 
 	rcu_read_lock();
 	list_for_each_entry_rcu(cb, &q->stats->callbacks, list) {
-		if (blk_stat_is_active(cb)) {
-			bucket = cb->bucket_fn(rq);
-			if (bucket < 0)
-				continue;
-			stat = &this_cpu_ptr(cb->cpu_stat)[bucket];
-			__blk_stat_add(stat, value);
-		}
+		if (!blk_stat_is_active(cb))
+			continue;
+
+		bucket = cb->bucket_fn(rq);
+		if (bucket < 0)
+			continue;
+
+		stat = &get_cpu_var(cb->cpu_stat)[bucket];
+		__blk_stat_add(stat, value);
+		put_cpu_var(cb->cpu_stat);
 	}
 	rcu_read_unlock();
 }

-- 
Jens Axboe