Re: [RFC] Badness in __mutex_unlock_slowpath with XFS stress tests

[Suzuki <suzuki@in.ibm.com>]

Hi Nathan,

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...

Yes, the debug_mutex_on gets switched off after the first occurence.

> 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?

But, what happens if __generic_file_aio_read() hits some error before 
doing the aops->direct_IO ?

> > 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.

Suzuki.
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