Re: [PATCH] btrfs: fix BUG_ON condition in btrfs_cancel_balance
From: David Sterba
Date: Thu Aug 17 2023 - 08:19:09 EST
On Tue, Aug 15, 2023 at 02:55:59AM -0400, xiaoshoukui wrote:
> Pausing and canceling balance can race to intterupt balance lead to BUG_ON
> panic in btrfs_cancel_balance. The BUG_ON condition in btrfs_cancel_balance
> does not take this race scenario into account.
Seems that it's from times the balance was not cancellable the same way
as now. Also it's a good time to switch the BUG_ON to an assertion or
handle it properly.
>
> However, the race condition has no other side effects. We can fix that.
>
> Reproducing it with panic trace like this:
> kernel BUG at fs/btrfs/volumes.c:4618!
> RIP: 0010:btrfs_cancel_balance+0x5cf/0x6a0
> Call Trace:
> <TASK>
> ? do_nanosleep+0x60/0x120
> ? hrtimer_nanosleep+0xb7/0x1a0
> ? sched_core_clone_cookie+0x70/0x70
> btrfs_ioctl_balance_ctl+0x55/0x70
> btrfs_ioctl+0xa46/0xd20
> __x64_sys_ioctl+0x7d/0xa0
> do_syscall_64+0x38/0x80
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
>
> Race scenario as follows:
> > mutex_unlock(&fs_info->balance_mutex);
> > --------------------
> > .......issue pause and cancel req in another thread
> > --------------------
> > ret = __btrfs_balance(fs_info);
> >
> > mutex_lock(&fs_info->balance_mutex);
> > if (ret == -ECANCELED && atomic_read(&fs_info->balance_pause_req)) {
> > btrfs_info(fs_info, "balance: paused");
> > btrfs_exclop_balance(fs_info, BTRFS_EXCLOP_BALANCE_PAUSED);
> > }
>
> Signed-off-by: xiaoshoukui <xiaoshoukui@xxxxxxxxxxxxx>
> ---
> fs/btrfs/volumes.c | 3 +--
> 1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 2ecb76cf3d91..886d667419ed 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -4638,8 +4638,7 @@ int btrfs_cancel_balance(struct btrfs_fs_info *fs_info)
> }
> }
>
> - BUG_ON(fs_info->balance_ctl ||
> - test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags));
> + BUG_ON(test_bit(BTRFS_FS_BALANCE_RUNNING, &fs_info->flags));
I'll change to to ASSERT, this is really to verify that the state
tracking works properly.
> atomic_dec(&fs_info->balance_cancel_req);
> mutex_unlock(&fs_info->balance_mutex);
> return 0;
> --
> 2.34.1