Re: [Xen-devel] [PATCH v6 1/5] x86/mem_sharing: reorder when pages are unlocked and released

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Thu, Jul 18, 2019 at 8:47 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> On 18.07.2019 16:35, Tamas K Lengyel wrote:
> > On Thu, Jul 18, 2019 at 8:28 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
> >> On 18.07.2019 15:47, Tamas K Lengyel wrote:
> >>> I feel like we are going in circles and having the same conversations
> >>> over and over without really making any headway. You introduced
> >>> grabbing the broken extra reference in 0502e0adae2. It is and was
> >>> actually unneeded to start with if the proper solution was put in
> >>> place, which is what this patch does, reordering things.
> >>
> >> I'm not complaining about the changes; I'd merely like the description
> >> state why they're needed.
> >
> > OK.
> >
> >>> It's possible there are other instances where this may still be
> >>> broken. Right now I only have bandwidth to test and fix the paths I
> >>> use. If that's unacceptable I'm happy to continue development in my
> >>> private fork and leave things as-is upstream.
> >>
> >> Similarly, if there are limitations - fine. But please say so in the
> >> description, to avoid giving the impression that the issues have been
> >> taken care of altogether.
> >
> > Fair enough.
>
> And btw - if you just sent an updated description, I think I'd commit
> this without further waiting for George to find time to eventually ack
> it.

That would be great. This is the message I typed out for v7:

    x86/mem_sharing: reorder when pages are unlocked and released

    Calling _put_page_type while also holding the page_lock for that
page can cause
    a deadlock. There may be code-paths still in place where this is
an issue, but
    for normal sharing purposes this has been tested and works.

    Removing grabbing the extra page reference at certain points is
done because it
    is no longer needed, a reference is held till necessary with this
reorder thus
    the extra reference is redundant.

    The comment being dropped is incorrect since it's now out-of-date.

Tamas

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