Re: [PATCH RESEND v4 2/8] btrfs: Make btrfs_parse_options() parse mount option in a atomic way

[Qu Wenruo]

-------- Original Message --------
Subject: Re: [PATCH RESEND v4 2/8] btrfs: Make btrfs_parse_options() 
parse mount option in a atomic way
From: Miao Xie <miao...@huawei.com>
To: Qu Wenruo <quwen...@cn.fujitsu.com>, <linux-btrfs@vger.kernel.org>
Date: 2015年01月30日 09:29
On Fri, 30 Jan 2015 09:20:46 +0800, Qu Wenruo wrote:
Here need ACCESS_ONCE to wrap info->mount_opt, or the complier might use
info->mount_opt instead of new_opt.
Thanks for pointing out this one.
But I worried that this is not key reason of the wrong space cache. Could
you explain the race condition which caused the wrong space cache?
Thanks
Miao
CPU0:
remount()
|- sync_fs() <- after sync_fs() we can start new trans
|- btrfs_parse_options()     CPU1:
                     |- start_transaction()
                     |- Do some bg allocation, not recorded in space_cache.
I think it is a bug if a free space is not recorded in space cache. Could you
explain why it is not recorded?

Thanks
Miao
IIRC, in that window, the fs_info->mount_opt's SPACE_CACHE bit is cleared.
So space cache is not recorded.

Thanks,
Qu

      |- set SPACE_CACHE bit due to cache_gen

                     |- commit_transaction()
                         |- write space cache and update cache_gen.
                             but since some of it is not recorded in space 
cache,
                             the space cache missing some records.
      |- clear SPACE_CACHE bit dut to nospace_cache

So the space cache is wrong.

Thanks,
Qu
+    }
       kfree(orig);
       return ret;
   }

.


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