Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on frozen fs to avoid deadlock.

[Qu Wenruo <quwenruo@cn.fujitsu.com>]

-------- Original Message --------
Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on 
frozen fs to avoid deadlock.
From: Chris Mason <clm@fb.com>
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
Date: 2015年01月21日 09:05
>
>
> On Tue, Jan 20, 2015 at 7:58 PM, Qu Wenruo <quwenruo@cn.fujitsu.com> 
> wrote:
>>
>> -------- Original Message --------
>> Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on 
>> frozen fs to avoid deadlock.
>> From: David Sterba <dsterba@suse.cz>
>> To: Qu Wenruo <quwenruo@cn.fujitsu.com>
>> Date: 2015年01月21日 01:13
>>> On Mon, Jan 19, 2015 at 03:42:41PM +0800, Qu Wenruo wrote:
>>>> --- a/fs/btrfs/super.c
>>>> +++ b/fs/btrfs/super.c
>>>> @@ -1000,6 +1000,14 @@ int btrfs_sync_fs(struct super_block *sb, 
>>>> int wait)
>>>>                */
>>>>               if (fs_info->pending_changes == 0)
>>>>                   return 0;
>>>> +            /*
>>>> +             * Test if the fs is frozen, or start_trasaction
>>>> +             * will deadlock on itself.
>>>> +             */
>>>> +            if (__sb_start_write(sb, SB_FREEZE_FS, false))
>>>> +                __sb_end_write(sb, SB_FREEZE_FS);
>>>> +            else
>>>> +                return 0;
>
> But what if someone freezes the FS after __sb_end_write() and before 
> btrfs_start_transaction()?   I don't see what keeps new freezers from 
> coming in.
>
> -chris
Either VFS::freeze_super() and VFS::syncfs() will hold the s_umount 
mutex, so freeze will not happen
during sync.

Thanks,
Qu
>
>