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SubjectRe: [PATCH v2 1/2] elevator: Fix a race in elevator switching and md device initialization
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Hi Tejun,

Thank you for the review.

On 9/22/13 13:04 , "Tejun Heo" <tj@kernel.org> wrote:

>On Fri, Aug 30, 2013 at 06:47:07PM -0400, Tomoki Sekiyama wrote:
>> @@ -739,9 +739,17 @@ blk_init_allocated_queue(struct request_queue *q,
>>request_fn_proc *rfn,
>>
>> q->sg_reserved_size = INT_MAX;
>>
>> + /* Protect q->elevator from elevator_change */
>> + mutex_lock(&q->sysfs_lock);
>> +
>> /* init elevator */
>> - if (elevator_init(q, NULL))
>> + if (elevator_init(q, NULL)) {
>> + mutex_unlock(&q->sysfs_lock);
>> return NULL;
>> + }
>> +
>> + mutex_unlock(&q->sysfs_lock);
>> +
>> return q;
>> }
>> EXPORT_SYMBOL(blk_init_allocated_queue);
>> diff --git a/block/elevator.c b/block/elevator.c
>> index 668394d..02d4390 100644
>> --- a/block/elevator.c
>> +++ b/block/elevator.c
>> @@ -186,6 +186,12 @@ int elevator_init(struct request_queue *q, char
>>*name)
>> struct elevator_type *e = NULL;
>> int err;
>>
>> + /*
>> + * q->sysfs_lock must be held to provide mutual exclusion between
>> + * elevator_switch() and here.
>> + */
>> + lockdep_assert_held(&q->sysfs_lock);
>> +
>> if (unlikely(q->elevator))
>> return 0;
>
>Hmm... why aren't we just changing elevator_init() to grab sysfs_lock
>where necessary?

The locking cannot be moved into elevator_init() because it is called
from elevator_switch() path, where the request_queue's sysfs_lock is
already taken.

> It'd be more consistent with elevator_exit() that way.

What elevator_exit() locks is elevator_queue's sysfs_lock, not
request_queue's sysfs_lock. What we need here is request_queue's
sysfs_lock.

>Thanks.
>
>--
>Tejun


Thanks,
Tomoki Sekiyama




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