Re: [PATCH] mm: Fix a NULL dereference crash while accessing bdev->bd_disk

[Jan Kara <jack@suse.cz>]

On Mon 28-11-16 10:57:18, Tejun Heo wrote:
> Hello, Jan.
> 
> On Mon, Nov 28, 2016 at 11:07:18AM +0100, Jan Kara wrote:
> > As I'm looking into the code, we need a serialization between bdev writeback
> > and blkdev_put(). That should be doable if we use writeback_single_inode()
> > for writing bdev inode instead of simple filemap_fdatawrite() and then use
> > inode_wait_for_writeback() in blkdev_put() but it needs some careful
> > thought.
> 
> It's kinda weird that sync() is ends up accessing bdev's without any
> synchronization.  Can't we just make iterate_bdevs() grab bd_mutex and
> verify bd_disk isn't NULL before calling into the callback?

This reminded me I've already seen something like this and indeed I've
already had a very similar discussion in March -
https://patchwork.kernel.org/patch/8556941/

Holding bd_mutex in iterate_devs() works but still nothing protects from
flusher thread just walking across the block device inode and trying to
write it which would result in the very same oops.

								Honza

-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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