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    SubjectRe: udf deadlock (was Re: hugetlbfs lockdep spew revisited.)
    On Fri 17-02-12 17:48:18, Al Viro wrote:
    > On Fri, Feb 17, 2012 at 12:49:22AM +0000, Al Viro wrote:
    > > Folks, this is not a false positive and it has nothing to do with misannotation
    > > for directories. Deadlock is real; I have no idea WTF do we what ->i_mutex
    > > held over that area in hugetlbfs ->mmap(), but doing that is really, really
    > > wrong, whatever the reason.
    >
    > Arrrrgh... Some grepping around has uncovered another deadlock on
    > i_mutex/mmap_sem and this one is not hard to hit at all.
    >
    > Thread A:
    > opens file on UDF (O_RDWR open)
    > does big, fat write() to it
    > Thread B:
    > opens the same file (also O_RDWR)
    > mmaps it
    > closes
    > does munmap()
    >
    > and there we are - munmap() will end up closing the second struct file,
    > call udf_release_file() and we are hitting ->i_mutex while under
    > ->mmap_sem. Blocking on it, actually, since generic_file_aio_write()
    > in the first thread is holding ->i_mutex. And as soon as thread A gets
    > around to faulting the next piece of data in, well... To widen the
    > window a lot, mmap something large sitting on NFS and do write() from
    > that mmapped area. Race window as wide as one could ask for...
    Right, I didn't realize ->release() may be called with mmap_sem held.
    Thanks for spotting this. BTW: Documentation/filesystems/Locking might
    need an update since it states:
    locking rules:
    All may block except for ->setlease.
    No VFS locks held on entry except for ->setlease.

    > What happens there is prealloc discard on close; do we even want ->i_mutex
    > there these days? Note that there's also
    > down_write(&UDF_I(inode)->i_data_sem);
    > in udf_release_file()...
    I've looked around and it seems we don't need i_mutex for anything.
    i_data_sem should be enough. So I'll just remove i_mutex.

    Honza
    --
    Jan Kara <jack@suse.cz>
    SUSE Labs, CR


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