Re: false positive lockdep splat with loop device

On Tue, Sep 26, 2017 at 7:24 AM, Dave Chinner [off-list ref] wrote:

On Thu, Sep 21, 2017 at 09:43:41AM +0300, Amir Goldstein wrote:

quoted

On Thu, Sep 21, 2017 at 1:22 AM, Dave Chinner [off-list ref] wrote:

quoted

[cc lkml, PeterZ and Byungchul]

...

quoted

The thing is, this IO completion has nothing to do with the lower
filesystem - it's the IO completion for the filesystem on the loop
device (the upper filesystem) and is not in any way related to the
IO completion from the dax device the lower filesystem is waiting
on.

IOWs, this is a false positive.

Peter, this is the sort of false positive I mentioned were likely to
occur without some serious work to annotate the IO stack to prevent
them.  We can nest multiple layers of IO completions and locking in
the IO stack via things like loop and RAID devices.  They can be
nested to arbitrary depths, too (e.g. loop on fs on loop on fs on
dm-raid on n * (loop on fs) on bdev) so this new completion lockdep
checking is going to be a source of false positives until there is
an effective (and simple!) way of providing context based completion
annotations to avoid them...

IMO, the way to handle this is to add 'nesting_depth' information
on blockdev (or bdi?). 'nesting' in the sense of blockdev->fs->blockdev->fs.
AFAIK, the only blockdev drivers that need to bump nesting_depth
are loop and nbd??

You're assumming that this sort of "completion inversion" can only
happen with bdev->fs->bdev, and that submit_bio_wait() is the only
place where completions are used in stackable block devices.

AFAICT, this could happen on with any block device that can be
stacked multiple times that uses completions. e.g. MD has a function
sync_page_io() that calls submit_bio_wait(), and that is called from
places in the raid 5, raid 10, raid 1 and bitmap layers (plus others
in DM). These can get stacked anywhere - even on top of loop devices
- and so I think the issue has a much wider scope than just loop and
nbd devices.

True. We can probably duplicate the struct file_system_type pattern,
something like:

struct block_device_type = loop_blkdev_type = {
        .owner          = THIS_MODULE,
        .name           = "loop",
        .devt             = MKDEV(LOOP_MAJOR, 0),
        .probe           = loop_probe,
        .lock              = NULL,
};

...
blk_register_region(&loop_blkdev_type, range, NULL);

And then we have a static place holder for lock_class_key address
to be used when annotating bio completions.
I realize same block device types can be nested via raid/dm/loop,
but as we can see in the splat so do same file system types via loop/nbd,
so we can think of similar solution to both cases.

quoted

Not sure if the kernel should limit loop blockdev nesting depth??

There's no way we should do that just because new lockdep
functionality is unable to express such constructs.

Of course not. I was just wondering if there should be a limit to
blockdev nesting regardless (e.g. too much queuing in the io stack).

But even if there is no limit to blockdev nesting, we can define
CONFIG_LOCKDEP_MAX_NESTING and:

struct lock_class_nested_key {
        struct lock_class_key     keys[CONFIG_LOCKDEP_MAX_NESTING];
};

Then struct file_system_type and struct block_device_type keys
could be of type struct lock_class_nested_key.

nested_depth should be carried in struct gendisk and the it is the
responsibility of  the blockdev driver to bump nested_depth if it
is a nested blockdev.

Callers of lockdep_set_class() ,like lockdep_annotate_inode_mutex_key()
should use a variant lockdep_set_class_nested(lock, ket, nested_depth)
if appropriate.

For any depth greater than CONFIG_LOCKDEP_MAX_NESTING,
lockdep_set_class_nested() will overload the last key in the
array, so we don't solve false positives for infinite nesting depth,
but we solve them for a defined max nesting depth.

Alas, even if this is workable, it will not be anytime soon that I can
backup this scribble with tested patches.

Amir.