Messages in this thread | | | Date | Mon, 10 Jul 2006 12:46:23 -0400 (EDT) | From | Stephane Doyon <> | Subject | Re: [RFC] Badness in __mutex_unlock_slowpath with XFS stress tests |
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Hi,
A few months back, a fix was introduced for a mutex double unlock warning related to direct I/O in XFS. I believe that fix has a lock ordering problem that can cause a deadlock.
The problem was that __blockdev_direct_IO() would unlock the i_mutex in the READ and DIO_OWN_LOCKING case, and the mutex would be unlocked again in xfs_read() soon after returning from __generic_file_aio_read(). Because there are lots of execution paths down from __generic_file_aio_read() that do not consistently release the i_mutex, the safest fix was deemed to be to reacquire the i_mutex before returning from __blockdev_direct_IO(). At this point however, the reader is holding an xfs ilock, and AFAICT the i_mutex is usually taken BEFORE the xfs ilock.
We are seeing a deadlock between a process writing and another process reading the same file, when the reader is using direct I/O. (The writer must apparently be growing the file, using either direct or buffered I/O.) Something like this, on XFS: (dd if=/dev/zero of=f bs=128K count=5000 & ) ; sleep 1 ; dd of=/dev/null if=f iflag=direct bs=128K count=5000
Seen on kernels 2.6.16 and 2.6.17.
The deadlock scenario appears to be this: -The reader goes into xfs_read(), takes the i_mutex and then an xfs_ilock XFS_IOLOCK_SHARED, then calls down to __generic_file_aio_read() which eventually goes down to __blockdev_direct_IO(). In there it drops the i_mutex. -The writer goes into xfs_write() and obtains the i_mutex. It then tries to get an xfs_ilock XFS_ILOCK_EXCL and must wait on it since it's held by the reader. -The reader, still in __blockdev_direct_IO(), executes directio_worker() and then tries to reacquire the i_mutex, and must wait on it because the writer has it.
And so we have a deadlock.
I leave it to the experts to figure out what the right fix for this might be. A workaround might be to not release the i_mutex during __blockdev_direct_IO().
Thanks
On Thu, 9 Mar 2006, Christoph Hellwig wrote:
> On Thu, Mar 09, 2006 at 01:24:38PM +0530, Suzuki wrote: >> >> Missed out linux-aio & linux-fs-devel lists. Forwarding. >> >> Comments ? > > I've seen this too. The problem is that __generic_file_aio_read can return > with or without the i_mutex locked in the direct I/O case for filesystems > that set DIO_OWN_LOCKING. It's a nasty one and I haven't found a better solution > than copying lots of code from filemap.c into xfs. > > >
On Fri, 10 Mar 2006, Nathan Scott wrote:
> On Thu, Mar 09, 2006 at 12:47:01PM +0530, Suzuki wrote: >> Hi all, > > Hi there Suzuki, > >> I was working on an issue with getting "Badness in >> __mutex_unlock_slowpath" and hence a stack trace, while running FS >> stress tests on XFS on 2.6.16-rc5 kernel. > > Thanks for looking into this. > >> The dmesg looks like : >> >> Badness in __mutex_unlock_slowpath at kernel/mutex.c:207 >> [<c0103c0c>] show_trace+0x20/0x22 >> [<c0103d4b>] dump_stack+0x1e/0x20 >> [<c0473f1f>] __mutex_unlock_slowpath+0x12a/0x23b >> [<c0473938>] mutex_unlock+0xb/0xd >> [<c02a5720>] xfs_read+0x230/0x2d9 >> [<c02a1bed>] linvfs_aio_read+0x8d/0x98 >> [<c015f3df>] do_sync_read+0xb8/0x107 >> [<c015f4f7>] vfs_read+0xc9/0x19b >> [<c015f8b2>] sys_read+0x47/0x6e >> [<c0102db7>] sysenter_past_esp+0x54/0x75 > > Yeah, test 008 from the xfstests suite was reliably hitting this for > me, it'd just not percolated to the top of my todo list yet. > >> This happens with XFS DIO reads. xfs_read holds the i_mutex and issues a >> __generic_file_aio_read(), which falls into __blockdev_direct_IO with >> DIO_OWN_LOCKING flag (since xfs uses own_locking ). Now >> __blockdev_direct_IO releases the i_mutex for READs with >> DIO_OWN_LOCKING.When it returns to xfs_read, it tries to unlock the >> i_mutex ( which is now already unlocked), causing the "Badness". > > Indeed. And there's the problem - why is XFS releasing i_mutex > for the direct read in xfs_read? Shouldn't be - fs/direct-io.c > will always release i_mutex for a direct read in the own-locking > case, so XFS shouldn't be doing it too (thats what the code does > and thats what the comment preceding __blockdev_direct_IO says). > > The only piece of the puzzle I don't understand is why we don't > always get that badness message at the end of every direct read. > Perhaps its some subtle fastpath/slowpath difference, or maybe > "debug_mutex_on" gets switched off after the first occurance... > > Anyway, with the above change (remove 2 lines near xfs_read end), > I can no longer reproduce the problem in that previously-warning > test case, and all the other XFS tests seem to be chugging along > OK (which includes a healthy mix of dio testing). > >> The possible solution which we can think of, is not to unlock the >> i_mutex for DIO_OWN_LOCKING. This will only affect the DIO_OWN_LOCKING >> users (as of now, only XFS ) with concurrent DIO sync read requests. AIO >> read requests would not suffer this problem since they would just return >> once the DIO is submitted. > > I don't think that level of invasiveness is necessary at this stage, > but perhaps you're seeing something that I've missed? Do you see > any reason why removing the xfs_read unlock wont work? > >> Another work around for this can be adding a check "mutex_is_locked" >> before trying to unlock i_mutex in xfs_read. But this seems to be an >> ugly hack. :( > > Hmm, that just plain wouldn't work - what if the lock was released > in generic direct IO code, and someone else had acquired it before > we got to the end of xfs_read? Badness for sure. > > cheers. > >
On Fri, 10 Mar 2006, Nathan Scott wrote:
> On Fri, Mar 10, 2006 at 10:14:22AM +1100, Nathan Scott wrote: >> On Thu, Mar 09, 2006 at 10:42:19PM +0000, Christoph Hellwig wrote: >>> On Fri, Mar 10, 2006 at 09:30:42AM +1100, Nathan Scott wrote: >>>> Not for reads AFAICT - __generic_file_aio_read + own-locking >>>> should always have released i_mutex at the end of the direct >>>> read - are you thinking of writes or have I missed something? >>> >>> if an error occurs before a_ops->direct_IO is called __generic_file_aio_read >>> will return with i_mutex still locked. Note that checking for negative >>> return values is not enough as __blockdev_direct_IO can return errors >>> aswell. >> >> *groan* - right you are. Another option may be to have the >> generic dio+own-locking case reacquire i_mutex if it drops >> it, before returning... thoughts? Seems alot less invasive >> than the filemap.c code dup'ing thing. > > Something like this (works OK for me)... > > cheers. > > - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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