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Date: Mon, 4 May 2015 15:29:37 +0900
From: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nitin Gupta <ngupta@vflare.org>, linux-kernel@vger.kernel.org,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [PATCHv3 9/9] zram: add dynamic device add/remove functionality
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On (05/04/15 11:20), Minchan Kim wrote:
> I had a time to think over it.
> 
> I think your patch is rather tricky so someone cannot see sysfs
> although he already opened /dev/zram but after a while he can see sysfs.
> It's weired.
> 
> I want to fix it more generic way. Othewise, we might have trouble with
> locking problem sometime. We already have experieced it with init_lock
> although we finally fixed it.
> 
> I think we can fix it with below patch I hope it's more general and right
> approach. It's based on your [zram: return zram device_id from zram_add()]
> 
> What do you think about?
> 

thanks for looking! didn't have much time over the weekend to
investigate. will take a look later today. at glance I think that
may do the trick.

	-ss

> From e943df5407b880f9262ef959b270226fdc81bc9f Mon Sep 17 00:00:00 2001
> From: Minchan Kim <minchan@kernel.org>
> Date: Mon, 4 May 2015 08:36:07 +0900
> Subject: [PATCH 1/2] zram: close race by open overriding
> 
> [1] introduced bdev->bd_mutex to protect a race between mount
> and reset. At that time, we don't have dynamic zram-add/remove
> feature so it was okay.
> 
> However, as we introduce dynamic device feature, bd_mutex became
> trouble.
> 
> 	CPU 0
> 
> echo 1 > /sys/block/zram<id>/reset
>   -> kernfs->s_active(A)
>     -> zram:reset_store->bd_mutex(B)
> 
> 	CPU 1
> 
> echo <id> > /sys/class/zram/zram-remove
>   ->zram:zram_remove: bd_mutex(B)
>   -> sysfs_remove_group
>     -> kernfs->s_active(A)
> 
> IOW, AB -> BA deadlock
> 
> The reason we are holding bd_mutex for zram_remove is to prevent
> any incoming open /dev/zram[0-9]. Otherwise, we could remove zram
> others already have opened. But it causes above deadlock problem.
> 
> To fix the problem, this patch overrides block_device.open and
> it returns -EBUSY if zram asserts he claims zram to reset so any
> incoming open will be failed so we don't need to hold bd_mutex
> for zram_remove ayn more.
> 
> This patch is to prepare for zram-add/remove feature.
> 
> [1] ba6b17: zram: fix umount-reset_store-mount race condition
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
>  drivers/block/zram/zram_drv.c | 42 ++++++++++++++++++++++++++++++++----------
>  drivers/block/zram/zram_drv.h |  4 ++++
>  2 files changed, 36 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
> index 3df4394..7fb72dc 100644
> --- a/drivers/block/zram/zram_drv.c
> +++ b/drivers/block/zram/zram_drv.c
> @@ -1074,13 +1074,6 @@ static ssize_t reset_store(struct device *dev,
>  	if (!bdev)
>  		return -ENOMEM;
>  
> -	mutex_lock(&bdev->bd_mutex);
> -	/* Do not reset an active device! */
> -	if (bdev->bd_openers) {
> -		ret = -EBUSY;
> -		goto out;
> -	}
> -
>  	ret = kstrtou16(buf, 10, &do_reset);
>  	if (ret)
>  		goto out;
> @@ -1090,23 +1083,52 @@ static ssize_t reset_store(struct device *dev,
>  		goto out;
>  	}
>  
> +	mutex_lock(&bdev->bd_mutex);
> +	/* Do not reset an active device or claimed device */
> +	if (bdev->bd_openers || zram->claim) {
> +		ret = -EBUSY;
> +		mutex_unlock(&bdev->bd_mutex);
> +		goto out;
> +	}
> +
> +	/* From now on, anyone can't open /dev/zram[0-9] */
> +	zram->claim = true;
> +	mutex_unlock(&bdev->bd_mutex);
> +
>  	/* Make sure all pending I/O is finished */
>  	fsync_bdev(bdev);
>  	zram_reset_device(zram);
>  
> -	mutex_unlock(&bdev->bd_mutex);
>  	revalidate_disk(zram->disk);
>  	bdput(bdev);
>  
> -	return len;
> +	mutex_lock(&bdev->bd_mutex);
> +	zram->claim = false;
> +	mutex_unlock(&bdev->bd_mutex);
>  
> +	return len;
>  out:
> -	mutex_unlock(&bdev->bd_mutex);
>  	bdput(bdev);
>  	return ret;
>  }
>  
> +static int zram_open(struct block_device *bdev, fmode_t mode)
> +{
> +	int ret = 0;
> +	struct zram *zram;
> +
> +	WARN_ON(!mutex_is_locked(&bdev->bd_mutex));
> +
> +	zram = bdev->bd_disk->private_data;
> +	/* zram was claimed to reset so open request fails */
> +	if (zram->claim)
> +		ret = -EBUSY;
> +
> +	return ret;
> +}
> +
>  static const struct block_device_operations zram_devops = {
> +	.open = zram_open,
>  	.swap_slot_free_notify = zram_slot_free_notify,
>  	.rw_page = zram_rw_page,
>  	.owner = THIS_MODULE
> diff --git a/drivers/block/zram/zram_drv.h b/drivers/block/zram/zram_drv.h
> index 042994e..6dbe2df 100644
> --- a/drivers/block/zram/zram_drv.h
> +++ b/drivers/block/zram/zram_drv.h
> @@ -115,5 +115,9 @@ struct zram {
>  	 */
>  	u64 disksize;	/* bytes */
>  	char compressor[10];
> +	/*
> +	 * zram is claimed so open request will be failed
> +	 */
> +	bool claim; /* Protected by bdev->bd_mutex */
>  };
>  #endif
> -- 
> 1.9.3
> 
> -- 
> Kind regards,
> Minchan Kim
>