From: Andreas Dilger Subject: Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock Date: Wed, 16 Feb 2011 22:13:53 -0700 Message-ID: <169849F1-1830-4771-9D38-6A39C8B46C90@dilger.ca> References: <20110207205325.FB6A.61FB500B@jp.fujitsu.com> <20110215160630.GH17313@quack.suse.cz> <20110215170352.GE4255@thunk.org> <20110215172954.GK17313@quack.suse.cz> <20110216081746.54d146d1.toshi.okajima@jp.fujitsu.com> <20110216145627.GB5592@quack.suse.cz> <4D5C9B1B.2050304@jp.fujitsu.com> Mime-Version: 1.0 (Apple Message framework v1082) Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 8BIT Cc: Jan Kara , Ted Ts'o , Masayoshi MIZUMA , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org To: toshi.okajima@jp.fujitsu.com Return-path: In-Reply-To: <4D5C9B1B.2050304@jp.fujitsu.com> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 2011-02-16, at 20:50, Toshiyuki Okajima wrote: > (2011/02/16 23:56), Jan Kara wrote: >> >>> I got a reproducer from Mizuma-san yesterday, and then I executed it on the kernel without a fixed patch. After an hour, I confirmed that this deadlock happened. >>> >>> However, on the kernel with a fixed patch, this deadlock doesn't still happen >>> after 12 hours passed. >>> >>> The patch for linux-2.6.38-rc4 is as follows: >>> --- >>> fs/fs-writeback.c | 2 +- >>> 1 files changed, 1 insertions(+), 1 deletions(-) >>> >>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c >>> index 59c6e49..1c9a05e 100644 >>> --- a/fs/fs-writeback.c >>> +++ b/fs/fs-writeback.c >>> @@ -456,7 +456,7 @@ static bool pin_sb_for_writeback(struct super_block *sb) >>> spin_unlock(&sb_lock); >>> >>> if (down_read_trylock(&sb->s_umount)) { >>> - if (sb->s_root) >>> + if (sb->s_frozen == SB_UNFROZEN && sb->s_root) >>> return true; >>> up_read(&sb->s_umount); This seems like a very low-risk fix. >> So this is something along the lines I thought but it actually won't work >> for example if sync(1) is run while the filesystem is frozen (that takes >> s_umount semaphore in a different place). And generally, I'm not convinced >> there are not other places that try to do IO while holding s_umount >> semaphore... > > OK. I understand. > > This code only fixes the case for the following path: > writeback_inodes_wb > -> ext4_da_writepages > -> ext4_journal_start_sb > -> vfs_check_frozen > But, the code doesn't fix the other cases. > > We must modify the local filesystem part in order to fix all cases...? It seems worthwhile to implement the low-risk fix that covers the common case, and if/when someone hits the rare 3-process case and/or submits a patch for it then that one will be fixed also. Cheers, Andreas