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Date: Thu, 17 Aug 2017 09:34:44 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Waiman Long <longman@...hat.com>
Cc: Jens Axboe <axboe@...nel.dk>,
Jeff Layton <jlayton@...chiereds.net>,
"J. Bruce Fields" <bfields@...ldses.org>,
Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v2] blktrace: Fix potentail deadlock between delete &
sysfs ops
On Wed, 16 Aug 2017 16:40:40 -0400
Waiman Long <longman@...hat.com> 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@...hat.com>
> ---
>
> 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)
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