Messages in this thread | | | Date | Fri, 10 Mar 2006 11:36:23 +0530 | From | Suzuki <> | Subject | Re: [RFC] Badness in __mutex_unlock_slowpath with XFS stress tests |
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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. >
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