Re: INFO: possible circular locking dependency detected 3.1.0-rc2-00190-g3210d19

[Josh Boyer]

On Mon, Aug 22, 2011 at 9:27 AM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Mon, Aug 22, 2011 at 02:16:21PM +0100, Al Viro wrote:
>> On Mon, Aug 22, 2011 at 09:09:14AM -0400, Josh Boyer wrote:
>>
>> > We've had a report of this on 3.0.1 as well.  Slightly different
>> > scenario and fs, but the locks in question are the same.
>> > https://bugzilla.redhat.com/show_bug.cgi?id=730998
>> >
>> > It seems that with CONFIG_PROVE_LOCKING on, might_fault will always
>> > attempt to grab mm->mmap_sem.  The common flow here is that getdents
>> > calls filldir, which calls copy_to_user, which is what is calling
>> > might_fault.
>> >
>> > Beyond that, I'm a bit over my head at the moment because I don't know
>> > if the VFS is right and we just need some more lockdep annotations or
>> > if there really is a problem.
>>
>> Don't grab ->i_mutex in ->evict_inode().  Why are you doing that, anyway?
>
> Note, BTW, that readdir() is a red herring here; there is a much more
> relevant reason for that ranking.  Namely, write() doing copy_from_user()
> when the file we are writing into has i_mutex held by us.  That can fault
> and in this case we have a non-directory inode.  While you can't have
> directory mmapped, regular files can be mmapped just fine.

So the lockdep report in the RHBZ (which now that I look at it
probably isn't the same as this report) seems to be doing a readdir
while find is trying to mmap, which is calling into
hugetlbfs_file_mmap and throwing the same deadlock warning.  Is that
like the scenario you are describing above?

josh
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