(cc'ing Jan and quoting the whole message)
On Wed, Mar 08, 2017 at 06:19:37PM -0800, Tahsin Erdogan wrote:
> __blkdev_put() could surprise writeback thread by detaching the
> wb object from an inode that hasn't cleared the I_SYNC flag yet.
> This causes a NULL pointer dereference as seen below:
>
> BUG: unable to handle kernel NULL pointer dereference at (null)
> IP: locked_inode_to_wb_and_lock_list+0x38/0x440
> PGD 0
> Oops: 0000 [#1] SMP
> CPU: 0 PID: 34 Comm: kworker/u8:1 Not tainted 4.11.0-rc1+ #202
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> Workqueue: writeback wb_workfn (flush-8:16)
> task: ffff88013aa780c0 task.stack: ffffc9000012c000
> RIP: 0010:locked_inode_to_wb_and_lock_list+0x38/0x440
> RSP: 0018:ffffc9000012fb70 EFLAGS: 00010202
> RAX: 0000000000000001 RBX: 0000000000000000 RCX: 0000000000000018
> RDX: ffff88013aa780c0 RSI: ffff880139a478f8 RDI: ffff88013aa788b8
> RBP: ffffc9000012fba0 R08: 0000000000000001 R09: 0000000000000000
> R10: 00000000969da8e2 R11: 0000000000000000 R12: ffff880139a47858
> R13: ffff880139a478e0 R14: ffff880139a478f8 R15: ffff8801371f4058
> FS: 0000000000000000(0000) GS:ffff88013ae00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 0000000000000000 CR3: 0000000001012000 CR4: 00000000000006f0
> Call Trace:
> writeback_sb_inodes+0x3e1/0x7a0
> __writeback_inodes_wb+0x87/0xc0
> wb_writeback+0x2e7/0x5c0
> wb_workfn+0x2d1/0x9c0
> process_one_work+0x1d3/0x620
> worker_thread+0x126/0x4a0
> kthread+0x10a/0x140
> ret_from_fork+0x2e/0x40
> RIP: locked_inode_to_wb_and_lock_list+0x38/0x440 RSP: ffffc9000012fb70
> CR2: 0000000000000000
> ---[ end trace e0ea8a2695f4c86c ]---
>
> Make __blkdev_put() wait for the I_SYNC flag to clear before detaching
> wb.
>
> Fixes: 43d1c0eb7e11 ("block: detach bdev inode from its wb in __blkdev_put()")
> Signed-off-by: Tahsin Erdogan <tahsin@xxxxxxxxxx>
> ---
> fs/block_dev.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 2eca00ec4370..70fb82fcedd0 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -95,7 +95,7 @@ void kill_bdev(struct block_device *bdev)
>
> invalidate_bh_lrus();
> truncate_inode_pages(mapping, 0);
> -}
> +}
> EXPORT_SYMBOL(kill_bdev);
>
> /* Invalidate clean unused buffers and pagecache. */
> @@ -617,13 +617,13 @@ static loff_t block_llseek(struct file *file, loff_t offset, int whence)
> inode_unlock(bd_inode);
> return retval;
> }
> -
> +
> int blkdev_fsync(struct file *filp, loff_t start, loff_t end, int datasync)
> {
> struct inode *bd_inode = bdev_file_inode(filp);
> struct block_device *bdev = I_BDEV(bd_inode);
> int error;
> -
> +
> error = filemap_write_and_wait_range(filp->f_mapping, start, end);
> if (error)
> return error;
> @@ -1038,7 +1038,7 @@ void bdput(struct block_device *bdev)
> }
>
> EXPORT_SYMBOL(bdput);
> -
> +
White line contaminations.
> static struct block_device *bd_acquire(struct inode *inode)
> {
> struct block_device *bdev;
> @@ -1880,7 +1880,10 @@ static void __blkdev_put(struct block_device *bdev, fmode_t mode, int for_part)
> * Detaching bdev inode from its wb in __destroy_inode()
> * is too late: the queue which embeds its bdi (along with
> * root wb) can be gone as soon as we put_disk() below.
> + * Before detaching wb, wait for any writeback activity for
> + * inode to settle.
> */
> + inode_wait_for_writeback(bdev->bd_inode);
> inode_detach_wb(bdev->bd_inode);
> }
> if (bdev->bd_contains == bdev) {
Given that there's write_inode_now(@sync == true) call right above,
I'm not sure how waiting for I_SYNC once more time would make a
difference. Can you please explain how to trigger the issue?
Thanks.
--
tejun