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Date: Thu, 17 Aug 2017 12:24:39 -0400
From: Waiman Long <longman@...hat.com>
To: Steven Rostedt <rostedt@...dmis.org>
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 08/17/2017 09:34 AM, Steven Rostedt wrote:
> 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.
The s_active isn't an actual lock. It is a reference count (kn->count)
on the sysfs (kernfs) file. Removal of a sysfs file, however, require
a wait until all the references are gone. The reference count is
treated like a rwsem using lockdep instrumentation code.
>> *** 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.
Any suggestion on how to make it better?
>> 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.
I am going to add a comment block here.
>> 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?
I am referring to blk_trace sysfs files which are used for enable
tracing of block operations.
>> + * 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.
Yes, that is what the bd_deleting flag is for.
I was thinking about failing the sysfs operation after a certain number
of trylock attempts, but then it will require changes to user space code
to handle the occasional failures. Finally I decided to fail it only
when a delete operation is in progress as all hopes are lost in this case.
>> + * 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
The root cause is the lock inversion under this circumstance. I think
modifying the blk_trace code has the least impact overall. I agree that
the code is ugly. If you have a better suggestion, I will certainly like
to hear it.
Cheers,
Longman
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