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    SubjectRe: highmem deadlock fix [was Re: VM in 2.4.10(+tweaks) vs. 2.4.9-ac14/15(+stuff)]
    On Thu, Sep 27, 2001 at 04:16:11PM -0700, Linus Torvalds wrote:
    > On Fri, 28 Sep 2001, Andrea Arcangeli wrote:
    > However, your patch is racy:
    > > --- 2.4.10aa2/fs/buffer.c.~1~ Wed Sep 26 18:45:29 2001
    > > +++ 2.4.10aa2/fs/buffer.c Fri Sep 28 00:04:44 2001
    > > @@ -194,6 +194,7 @@
    > > struct buffer_head * bh = *array++;
    > > bh->b_end_io = end_buffer_io_sync;
    > > submit_bh(WRITE, bh);
    > > + clear_bit(BH_Pending_IO, &bh->b_state);
    > No way can we clear the bit here, because the submit_bh() may have caused
    > the buffer to be unlocked and IO to have completed, and it is no longer
    > "owned" by us - somebody else might have started IO on it and we'd be
    > clearing the bit for the wrong user.

    Moving clear_bit just above submit_bh will fix it (please Robert make
    this change before testing it), because if we block in submit_bh in the
    bounce, then we won't deadlock on ourself because of the pagehighmem
    check, and all previous non-pending bh are ok too, (only the next are
    problematic, and they're still marked pending_IO so we can't deadlock on

    So you can re-consider my approch, the design of the fix was ok, it was
    just a silly implementation error.

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