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Date: Tue, 20 Jan 2015 20:10:56 -0500
From: Chris Mason <clm@fb.com>
Subject: Re: [PATCH] btrfs: Don't call btrfs_start_transaction() on frozen
 fs to avoid deadlock.
To: Qu Wenruo <quwenruo@cn.fujitsu.com>
CC: <dsterba@suse.cz>, <linux-btrfs@vger.kernel.org>, <miaoxie@huawei.com>
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On Tue, Jan 20, 2015 at 8:09 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: 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.

You're right.  I was worried about the sync ioctl, but the mutex won't 
be held there to deadlock against.  We'll be fine.

-chris