Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb
From: Darrick J. Wong <hidden>
Date: 2021-01-04 21:59:04
Also in:
lkml
On Thu, Dec 24, 2020 at 04:49:54AM +0000, Satya Tangirala wrote:
freeze/thaw_bdev() currently use bdev->bd_fsfreeze_count to infer
whether or not bdev->bd_fsfreeze_sb is valid (it's valid iff
bd_fsfreeze_count is non-zero). thaw_bdev() doesn't nullify
bd_fsfreeze_sb.
But this means a freeze_bdev() call followed by a thaw_bdev() call can
leave bd_fsfreeze_sb with a non-null value, while bd_fsfreeze_count is
zero. If freeze_bdev() is called again, and this time
get_active_super() returns NULL (e.g. because the FS is unmounted),
we'll end up with bd_fsfreeze_count > 0, but bd_fsfreeze_sb is
*untouched* - it stays the same (now garbage) value. A subsequent
thaw_bdev() will decide that the bd_fsfreeze_sb value is legitimate
(since bd_fsfreeze_count > 0), and attempt to use it.
Fix this by always setting bd_fsfreeze_sb to NULL when
bd_fsfreeze_count is successfully decremented to 0 in thaw_sb().
Alternatively, we could set bd_fsfreeze_sb to whatever
get_active_super() returns in freeze_bdev() whenever bd_fsfreeze_count
is successfully incremented to 1 from 0 (which can be achieved cleanly
by moving the line currently setting bd_fsfreeze_sb to immediately
after the "sync:" label, but it might be a little too subtle/easily
overlooked in future).
This fixes the currently panicking xfstests generic/085.
Fixes: 040f04bd2e82 ("fs: simplify freeze_bdev/thaw_bdev")
Signed-off-by: Satya Tangirala <redacted>I came up with the same solution to the same crash, so: Reviewed-by: Darrick J. Wong <redacted> --D
quoted hunk ↗ jump to hunk
--- fs/block_dev.c | 2 ++ 1 file changed, 2 insertions(+)diff --git a/fs/block_dev.c b/fs/block_dev.c index 9e56ee1f2652..12a811a9ae4b 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c@@ -606,6 +606,8 @@ int thaw_bdev(struct block_device *bdev) error = thaw_super(sb); if (error) bdev->bd_fsfreeze_count++; + else + bdev->bd_fsfreeze_sb = NULL; out: mutex_unlock(&bdev->bd_fsfreeze_mutex); return error;-- 2.29.2.729.g45daf8777d-goog