On Fri, 30 Jan 2015 09:33:17 +0800, Qu Wenruo wrote: > > -------- 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.
SPACE_CACHE is used to control cache write out, not in-memory cache. All the free space should be recorded in in-memory cache.And when we write out the in-memory space cache, we need protect the space cache from changing. Thanks Miao > > 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; >>>>> } >>>>> >>> . >>> > > -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
