Re: [PATCH 2/2] quota: Fix deadlock with suspend and quotas

[Eric Sandeen <sandeen@sandeen.net>]

On 2/7/12 4:37 PM, Jan Kara wrote:
> This script causes a kernel deadlock:
> set -e
> DEVICE=/dev/vg1/linear
> lvchange -ay $DEVICE
> mkfs.ext3 $DEVICE
> mount -t ext3 -o usrquota,grpquota $DEVICE /mnt/test
> quotacheck -gu /mnt/test
> umount /mnt/test
> mount -t ext3 -o usrquota,grpquota $DEVICE /mnt/test
> quotaon /mnt/test
> dmsetup suspend $DEVICE
> setquota -u root 1 2 3 4 /mnt/test &
> sleep 1
> dmsetup resume $DEVICE
> 
> setquota acquired semaphore s_umount for read and then tried to perform a
> transaction (and waits because the device is suspended).  dmsetup resume tries
> to acquire s_umount for write before resuming the device (and waits for
> setquota).
> 
> Fix the deadlock by grabbing a thawed superblock for quota commands which need
> it.
> 
> Reported-by: Mikulas Patocka <mpatocka@redhat.com>
> Signed-off-by: Jan Kara <jack@suse.cz>

Makes sense to me, thanks.

Reviewed-by: Eric Sandeen <sandeen@redhat.com>

> ---
>  fs/quota/quota.c |   24 +++++++++++++++++++++---
>  1 files changed, 21 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/quota/quota.c b/fs/quota/quota.c
> index 7898cd6..fc2c438 100644
> --- a/fs/quota/quota.c
> +++ b/fs/quota/quota.c
> @@ -292,11 +292,26 @@ static int do_quotactl(struct super_block *sb, int type, int cmd, qid_t id,
>  	}
>  }
>  
> +/* Return 1 if 'cmd' will block on frozen filesystem */
> +static int quotactl_cmd_write(int cmd)
> +{
> +	switch (cmd) {
> +	case Q_GETFMT:
> +	case Q_GETINFO:
> +	case Q_SYNC:
> +	case Q_XGETQSTAT:
> +	case Q_XGETQUOTA:
> +	case Q_XQUOTASYNC:
> +		return 0;
> +	}
> +	return 1;
> +}
> +
>  /*
>   * look up a superblock on which quota ops will be performed
>   * - use the name of a block device to find the superblock thereon
>   */
> -static struct super_block *quotactl_block(const char __user *special)
> +static struct super_block *quotactl_block(const char __user *special, int cmd)
>  {
>  #ifdef CONFIG_BLOCK
>  	struct block_device *bdev;
> @@ -309,7 +324,10 @@ static struct super_block *quotactl_block(const char __user *special)
>  	putname(tmp);
>  	if (IS_ERR(bdev))
>  		return ERR_CAST(bdev);
> -	sb = get_super(bdev);
> +	if (quotactl_cmd_write(cmd))
> +		sb = get_super_thawed(bdev);
> +	else
> +		sb = get_super(bdev);
>  	bdput(bdev);
>  	if (!sb)
>  		return ERR_PTR(-ENODEV);
> @@ -361,7 +379,7 @@ SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
>  			pathp = &path;
>  	}
>  
> -	sb = quotactl_block(special);
> +	sb = quotactl_block(special, cmds);
>  	if (IS_ERR(sb)) {
>  		ret = PTR_ERR(sb);
>  		goto out;