Re: [Xen-devel] [PATCH 4/5] x86/mem_sharing: check for errors in p2m->set_entry().

[Tim Deegan <tim@xxxxxxx>]

At 10:07 -0500 on 07 Mar (1362650835), Andres Lagar-Cavilla wrote:
> On Mar 7, 2013, at 9:53 AM, Tim Deegan <tim@xxxxxxx> wrote:
> 
> > This call ought always to succeed.  Assert that it does rather than
> > ignoring the return value.
> > 
> > Signed-off-by: Tim Deegan <tim@xxxxxxx>
> > Cc: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>
> > ---
> > xen/arch/x86/mm/mem_sharing.c |   12 ++++++++----
> > 1 file changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/xen/arch/x86/mm/mem_sharing.c b/xen/arch/x86/mm/mem_sharing.c
> > index 1caa900..0364bb0 100644
> > --- a/xen/arch/x86/mm/mem_sharing.c
> > +++ b/xen/arch/x86/mm/mem_sharing.c
> > @@ -1273,6 +1273,8 @@ int relinquish_shared_pages(struct domain *d)
> >         p2m_access_t a;
> >         p2m_type_t t;
> >         mfn_t mfn;
> > +        int set_rc;
> > +
> >         if ( atomic_read(&d->shr_pages) == 0 )
> >             break;
> >         mfn = p2m->get_entry(p2m, gfn, &t, &a, 0, NULL);
> > @@ -1281,10 +1283,12 @@ int relinquish_shared_pages(struct domain *d)
> >             /* Does not fail with ENOMEM given the DESTROY flag */
> >             BUG_ON(__mem_sharing_unshare_page(d, gfn, 
> >                     MEM_SHARING_DESTROY_GFN));
> > -            /* Clear out the p2m entry so no one else may try to 
> > -             * unshare */
> > -            p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
> > -                            p2m_invalid, p2m_access_rwx);
> > +            /* Clear out the p2m entry so no one else may try to
> > +             * unshare.  Must succeed: we just read the old entry and
> > +             * we hold the p2m lock. */
> > +            set_rc = p2m->set_entry(p2m, gfn, _mfn(0), PAGE_ORDER_4K,
> > +                                    p2m_invalid, p2m_access_rwx);
> > +            ASSERT(set_rc != 0);
> Acked-by: Andres Lagar-Cavilla <andres@xxxxxxxxxxxxxxxx>

Thanks.

> Wouldn't it be slightly cleaner to BUG_ON(p2m->set_entry(..) != 1)?

I dislike BUG_ON(something_with_side_effects()).  When scanning though
code, my eye skips over ASSERT()s and BUG_ON()s, assuming they're just
testing invariants.  Besides, that sort of thinking leads to the much
more pernicious ASSERT(thing_with_side_effects). :)

Tim.

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