Re: [PATCH v2] blktrace: Fix potentail deadlock between delete & sysfs ops

[Steven Rostedt]

On Wed, 16 Aug 2017 16:40:40 -0400
Waiman Long <longman@xxxxxxxxxx> wrote:

> The lockdep code had reported the following unsafe locking scenario:
> 
>        CPU0                    CPU1
>        ----                    ----
>   lock(s_active#228);
>                                lock(&bdev->bd_mutex/1);
>                                lock(s_active#228);
>   lock(&bdev->bd_mutex);

Can you show the exact locations of these locks. I have no idea where
this "s_active" is.

> 
>  *** DEADLOCK ***
> 
> The deadlock may happen when one task (CPU1) is trying to delete
> a partition in a block device and another task (CPU0) is accessing
> tracing sysfs file in that partition.
> 
> To avoid that, accessing tracing sysfs file will now use a mutex
> trylock loop and the operation will fail if a delete operation is
> in progress.
> 
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> ---
> 
>  v2:
>    - Use READ_ONCE() and smp_store_mb() to read and write bd_deleting.
>    - Check for signal in the mutex_trylock loops.
>    - Use usleep() instead of schedule() for RT tasks.

I'm sorry but I really do hate this patch.

> 
>  block/ioctl.c           |  3 +++
>  include/linux/fs.h      |  1 +
>  kernel/trace/blktrace.c | 39 +++++++++++++++++++++++++++++++++++++--
>  3 files changed, 41 insertions(+), 2 deletions(-)
> 
> diff --git a/block/ioctl.c b/block/ioctl.c
> index 0de02ee..b920329 100644
> --- a/block/ioctl.c
> +++ b/block/ioctl.c
> @@ -86,12 +86,15 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
>  				return -EBUSY;
>  			}
>  			/* all seems OK */
> +			smp_store_mb(bdev->bd_deleting, 1);

No comment to explain what is happening here, and why.

>  			fsync_bdev(bdevp);
>  			invalidate_bdev(bdevp);
>  
>  			mutex_lock_nested(&bdev->bd_mutex, 1);
>  			delete_partition(disk, partno);
>  			mutex_unlock(&bdev->bd_mutex);
> +			smp_store_mb(bdev->bd_deleting, 0);
> +

ditto.

>  			mutex_unlock(&bdevp->bd_mutex);
>  			bdput(bdevp);
>  
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 6e1fd5d..c2ba35e 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -427,6 +427,7 @@ struct block_device {
>  #endif
>  	struct block_device *	bd_contains;
>  	unsigned		bd_block_size;
> +	int			bd_deleting;
>  	struct hd_struct *	bd_part;
>  	/* number of times partitions within this device have been opened. */
>  	unsigned		bd_part_count;
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index bc364f8..b2dffa9 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -27,6 +27,8 @@
>  #include <linux/time.h>
>  #include <linux/uaccess.h>
>  #include <linux/list.h>
> +#include <linux/delay.h>
> +#include <linux/sched/rt.h>
>  
>  #include "../../block/blk.h"
>  
> @@ -1605,6 +1607,23 @@ static struct request_queue *blk_trace_get_queue(struct block_device *bdev)
>  	return bdev_get_queue(bdev);
>  }
>  
> +/*
> + * Read/write to the tracing sysfs file requires taking references to the

What's the "tracing sysfs" file? tracefs?

> + * sysfs file and then acquiring the bd_mutex. Deleting a block device
> + * requires acquiring the bd_mutex and then waiting for all the sysfs
> + * references to be gone. This can lead to deadlock if both operations
> + * happen simultaneously. To avoid this problem, read/write to the
> + * the tracing sysfs files can now fail if the bd_mutex cannot be
> + * acquired while a deletion operation is in progress.
> + *
> + * A mutex trylock loop is used assuming that tracing sysfs operations

A mutex trylock loop is not enough to stop a deadlock. But I'm guessing
the undocumented bd_deleting may prevent that.

> + * aren't frequently enough to cause any contention problem.
> + *
> + * For RT tasks, a running high priority task will prevent any lower
> + * priority RT tasks from being run. So they do need to actually sleep
> + * when the trylock fails to allow lower priority tasks to make forward
> + * progress.
> + */
>  static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
>  					 struct device_attribute *attr,
>  					 char *buf)
> @@ -1622,7 +1641,15 @@ static ssize_t sysfs_blk_trace_attr_show(struct device *dev,
>  	if (q == NULL)
>  		goto out_bdput;
>  
> -	mutex_lock(&bdev->bd_mutex);
> +	while (!mutex_trylock(&bdev->bd_mutex)) {
> +		if (READ_ONCE(bdev->bd_deleting))
> +			goto out_bdput;
> +		if (signal_pending(current)) {
> +			ret = -EINTR;
> +			goto out_bdput;
> +		}
> +		rt_task(current) ? usleep_range(10, 10) : schedule();

We need to come up with a better solution. This is just a hack that
circumvents a lot of the lockdep infrastructure.

-- Steve

> +	}
>  
>  	if (attr == &dev_attr_enable) {
>  		ret = sprintf(buf, "%u\n", !!q->blk_trace);
> @@ -1683,7 +1710,15 @@ static ssize_t sysfs_blk_trace_attr_store(struct device *dev,
>  	if (q == NULL)
>  		goto out_bdput;
>  
> -	mutex_lock(&bdev->bd_mutex);
> +	while (!mutex_trylock(&bdev->bd_mutex)) {
> +		if (READ_ONCE(bdev->bd_deleting))
> +			goto out_bdput;
> +		if (signal_pending(current)) {
> +			ret = -EINTR;
> +			goto out_bdput;
> +		}
> +		rt_task(current) ? usleep_range(10, 10) : schedule();
> +	}
>  
>  	if (attr == &dev_attr_enable) {
>  		if (value)