Re: [Nbd] [PATCH 1/6] block: export blkdev_reread_part()

[Ming Lei <ming.lei@...1301...>]

On Mon, Apr 6, 2015 at 12:12 AM, Christoph Hellwig <hch@...1270...> wrote:
>> +/*
>> + * This is exported as API for block driver, can be called
>> + * with requiring bd_mutex or not.
>> + */
>> +int __blkdev_reread_part(struct block_device *bdev, bool lock)
>>  {
>>       struct gendisk *disk = bdev->bd_disk;
>>       int res;
>> @@ -159,12 +163,14 @@ static int blkdev_reread_part(struct block_device *bdev)
>>               return -EINVAL;
>>       if (!capable(CAP_SYS_ADMIN))
>>               return -EACCES;
>> -     if (!mutex_trylock(&bdev->bd_mutex))
>> +     if (lock && !mutex_trylock(&bdev->bd_mutex))
>>               return -EBUSY;
>
> Please don't add funtions that do conditional locking, instead move
> all the code into blkdev_reread_part_nolock, and then wrap it:
>
> int blkdev_reread_part(struct block_device *bdev)
> {
>         if (!mutex_trylock(&bdev->bd_mutex))
>                 return -EBUSY;
>         blkdev_reread_part_nolock(bdev);
>         mutex_unlock(&bdev->bd_mutex);
> }

Yes, it is more clean, but with extra acquiring lock cost for the
failure cases, especially when we replace trylock with mutex_lock().

>
> Please also add a lockdep_assert_held to blkdev_reread_part_nolock to
> ensure callers actually do hold the lock.

Good point!

Thanks,
Ming Lei