Re: [patch 0/9] buffered write deadlock fix

[Nick Piggin]

On Tue, Jan 30, 2007 at 03:21:19PM -0800, Andrew Morton wrote:
> On Tue, 30 Jan 2007 12:55:58 -0800
> Andrew Morton <akpm@xxxxxxxx> wrote:
> 
> > y'know, four or five years back I fixed this bug by doing
> > 
> > 	current->locked_page = page;
> > 
> > in the write() code, and then teaching the pagefault code to avoid locking
> > the same page.  Patch below.
> > 
> > But then evil mean Hugh pointed out that the patch is still vulnerable to
> > ab/ba deadlocking so I dropped it.
> 
> And he was right, of course.  Task A holds file a's i_mutex and takes a
> fault against file b's page.  Task B holds file b's i_mutex and takes a
> fault against file a's page.  Drat.
> 
> I wonder if there's a sane way of preventing that.

If you want to go down the path of carrying state around in task_struct,
you can take the mmap_sem and set a flag, then get_user_pages the source
page and lock both source and destination in ascending order, then your
page fault handler checks the flag and skips mmap_sem, and the rest of
your fault path checks both the page locks you're holding.

At which point you arrive at a horrible mess :)

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