From: Eric Sandeen Subject: Re: [RFC][PATCH] Re: [BUG] ext4: cannot unfreeze a filesystem due to a deadlock Date: Mon, 02 May 2011 09:04:23 -0500 Message-ID: <4DBEB9E7.9070407@redhat.com> References: <20110331234050.GD2904@dastard> <20110401140856.GA5311@quack.suse.cz> <20110406054005.GD31057@dastard> <20110406061856.GC23285@quack.suse.cz> <20110406112135.GE31057@dastard> <4DBE746F.3090707@ubuntu.com> <20110502105629.GA4556@quack.suse.cz> <4DBE9537.4050708@ubuntu.com> <20110502122055.GB5855@quack.suse.cz> <4DBEA3DF.1060306@ubuntu.com> <20110502131619.GC5855@quack.suse.cz> <4DBEB025.90704@ubuntu.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: Jan Kara , Dave Chinner , Toshiyuki Okajima , "Ted Ts'o" , Masayoshi MIZUMA , Andreas Dilger , linux-ext4@vger.kernel.org, linux-fsdevel@vger.kernel.org, Christoph Hellwig To: surbhi.palande@ubuntu.com Return-path: In-Reply-To: <4DBEB025.90704@ubuntu.com> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 5/2/11 8:22 AM, Surbhi Palande wrote: > On 05/02/2011 04:16 PM, Jan Kara wrote: >> On Mon 02-05-11 15:30:23, Surbhi Palande wrote: >>> On 05/02/2011 03:20 PM, Jan Kara wrote: >>>> On Mon 02-05-11 14:27:51, Surbhi Palande wrote: >>>>> On 05/02/2011 01:56 PM, Jan Kara wrote: >>>>>> On Mon 02-05-11 12:07:59, Surbhi Palande wrote: >>>>>>> On 04/06/2011 02:21 PM, Dave Chinner wrote: >>>>>>>> On Wed, Apr 06, 2011 at 08:18:56AM +0200, Jan Kara wrote: >>>>>>>>> On Wed 06-04-11 15:40:05, Dave Chinner wrote: >>>>>>>>>> On Fri, Apr 01, 2011 at 04:08:56PM +0200, Jan Kara wrote: >>>>>>>>>>> On Fri 01-04-11 10:40:50, Dave Chinner wrote: >>>>>>>>>>>> If you don't allow the page to be dirtied in the fist place, then >>>>>>>>>>>> nothing needs to be done to the writeback path because there is >>>>>>>>>>>> nothing dirty for it to write back. >>>>>>>>>>> Sure but that's only the problem he was able to hit. But generally, >>>>>>>>>>> there's a problem with needing s_umount for unfreezing because it isn't >>>>>>>>>>> clear there aren't other code paths which can block with s_umount held >>>>>>>>>>> waiting for fs to get unfrozen. And these code paths would cause the same >>>>>>>>>>> deadlock. That's why I chose to get rid of s_umount during thawing. >>>>>>>>>> Holding the s_umount lock while checking if frozen and sleeping >>>>>>>>>> is essentially an ABBA lock inversion bug that can bite in many more >>>>>>>>>> places that just thawing the filesystem. Any where this is done should >>>>>>>>>> be fixed, so I don't think just removing the s_umount lock from the thaw >>>>>>>>>> path is sufficient to avoid problems. >>>>>>>>> That's easily said but hard to do - any transaction start in ext3/4 may >>>>>>>>> block on filesystem being frozen (this seems to be similar for XFS as I'm >>>>>>>>> looking into the code) and transaction start traditionally nests inside >>>>>>>>> s_umount (and basically there's no way around that since sync() calls your >>>>>>>>> fs code with s_umount held). >>>>>>>> Sure, but the question must be asked - why is ext3/4 even starting a >>>>>>>> transaction on a clean filesystem during sync? A frozen filesystem, >>>>>>>> by definition, is a clean filesytem, and therefore sync calls of any >>>>>>>> kind should not be trying to write to the FS or start transactions. >>>>>>>> XFS does this just fine, so I'd consider such behaviour on a frozen >>>>>>>> filesystem a bug in ext3/4... >>>>>>> I had a look at the xfs code for seeing how this is done. >>>>>>> xfs_file_aio_write() >>>>>>> xfs_wait_for_freeze() >>>>>>> vfs_check_frozen() >>>>>>> So xfs_file_aio_write() writes to buffers when the FS is not frozen. >>>>>>> >>>>>>> Now, I want to know what stops the following scenario from happening: >>>>>>> -------------------- >>>>>>> xfs_file_aio_write() >>>>>>> xfs_wait_for_freeze() >>>>>>> vfs_check_frozen() >>>>>>> At this point F.S was not frozen, so the next instruction in the >>>>>>> xfs_file_aio_write() will be executed next. >>>>>>> However at this point (i.e after checking if F.S is frozen) the >>>>>>> write process gets pre-empted and say the _freeze_ process gets >>>>>>> control. >>>>>>> >>>>>>> Now the F.S freezes and the write process gets the control back. And >>>>>>> so we end up writing to the page cache when the F.S is frozen. >>>>>>> -------------------- >>>>>>> >>>>>>> Can anyone please enlighten me on how& why this premption is _not_ >>>>>>> possible? >>>>> Thanks for your reply. >>>>>> XFS works similarly as ext4 in this regard I believe. They have the log >>>>>> frozen in xfs_freeze() so if the race you describe above happens, either >>>>>> the writing process gets caught waiting for log to unfreeze >>>>> Agreed. >>>>>> or it manages >>>>>> to start a transaction and then freezing process waits for transaction to >>>>>> finish before it can proceed with freezing. I'm not sure why is there the >>>>>> check in xfs_file_aio_write()... >>>>>> >>>>>> >>>>> I am sorry, but I don't understand how this will happen - i.e I >>>>> can't understand what stops freeze_super() (or ext4_freeze) from >>>>> freezing a superblock (as the write process stopped just before >>>>> writing anything for this transaction and has not taken any locks?) >>>> So ext4_freeze() does >>>> jbd2_journal_lock_updates(journal) >>>> which waits for all running transactions to finish and updates >>>> j_barrier_count which stops any news ones from proceeding (check >>>> function start_this_handle()). >>>> >>> Yes, but ext4_freeze() also calls >>> jbd2_journal_unlock_updates(journal) which decrements the >>> j_barrier_count (which was previously updated/incremented in >>> jbd2_journal_lock_updates) ? before it returns. So after this call a >>> new transaction/handle can be accepted/started. >>> >>> A comment in ext4_freeze() says: >>> /* we rely on s_frozen to stop further updates */ >>> (before calling jbd2_journal_unlock_updates()) >> Ah, drat, you're right. I've missed this other part. It's the problem >> that if you expect to see something, you'll see it regardless of the real >> code ;). >> >> The fact is we do vfs_check_frozen() in ext4_journal_start_sb() but indeed >> it's still racy (although the race window is relatively small) because the >> filesystem can become frozen the instant after we check vfs_check_frozen(). >> Commit 6b0310fb broke it for ext4. >> >> I guess the code was mostly copied from XFS which seems to have the same >> problem in xfs_trans_alloc() since the git history beginning. I see two >> ways to fix this - either fix ext4/xfs to check s_frozen after starting >> a transaction and if the filesystem is being frozen, we stop the >> transaction, wait for fs to get unfrozen, and restart. Another option is >> to create an analogous logic using a atomic counter of write ops in vfs >> that could be used by all filesystems. We'd just have to replace >> vfs_check_frozen() with vfs_start_write() and add vfs_stop_write() at >> appropriate places... > How about calling jbd2_journal_unlock_updates(EXT4_SB(sb)->s_journal); > from ext4_unfreeze()? we used to have that, but holding it locked until then means we exit the kernel with a mutex held, which is pretty icky. ================================================ [ BUG: lock held when returning to user space! ] ------------------------------------------------ lvcreate/1075 is leaving the kernel with locks still held! 1 lock held by lvcreate/1075: #0: (&journal->j_barrier){+.+...}, at: [] jbd2_journal_lock_updates+0xe1/0xf0 -Eric > So that indeed no transactions can be started before unfreeze is called. > > This has another advantage, that it rightfully does not let you update the access time when the F.S is frozen (touch_atime called from a read path when the F.S is frozen) Otherwise we also need to fix this path. > > Warm Regards, > Surbhi. > >> Dave, Christoph, any opinions on this? >> Honza > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html