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Date: Mon, 4 Jan 2021 13:58:02 -0800 From: "Darrick J. Wong" <darrick.wong@...cle.com> To: Satya Tangirala <satyat@...gle.com> Cc: Alexander Viro <viro@...iv.linux.org.uk>, Christoph Hellwig <hch@....de>, linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] fs: Fix freeze_bdev()/thaw_bdev() accounting of bd_fsfreeze_sb 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 <satyat@...gle.com> I came up with the same solution to the same crash, so: Reviewed-by: Darrick J. Wong <darrick.wong@...cle.com> --D > --- > 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 >
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