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

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