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SubjectRe: highmem deadlock fix [was Re: VM in 2.4.10(+tweaks) vs. 2.4.9-ac14/15(+stuff)]

On Fri, 28 Sep 2001, Andrea Arcangeli wrote:
> 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

We won't block on _ourselves_, but we can block on _two_ people doing it,
and blocking on each others requests that are blocked waiting on a bounce
buffer. Both will have one locked buffer, both will be waiting for the
other person unlocking that buffer, and neither will ever make progress.

You could clear that bit _after_ the bounce buffer allocation, I suspect.

But I also suspect that it doesn't matter much, and as I can imagine
similar problems with GFP_NOIO and loopback etc (do you see any reason why
loopback couldn't deadlock on waiting for itself?), I think the GFP_XXX
thing is the proper fix.


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