[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Ping: [PATCH v2] x86/PoD: move increment of entry count
On 04.01.2022 11:57, Jan Beulich wrote: > When not holding the PoD lock across the entire region covering P2M > update and stats update, the entry count should indicate too large a > value in preference to a too small one, to avoid functions bailing early > when they find the count is zero. Hence increments should happen ahead > of P2M updates, while decrements should happen only after. Deal with the > one place where this hasn't been the case yet. > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx> > --- > v2: Add comments. Ping? Jan > --- > While it might be possible to hold the PoD lock over the entire > operation, I didn't want to chance introducing a lock order violation on > a perhaps rarely taken code path. > > --- a/xen/arch/x86/mm/p2m-pod.c > +++ b/xen/arch/x86/mm/p2m-pod.c > @@ -1342,19 +1342,22 @@ mark_populate_on_demand(struct domain *d > } > } > > + /* > + * Without holding the PoD lock across the entire operation, bump the > + * entry count up front assuming success of p2m_set_entry(), undoing the > + * bump as necessary upon failure. Bumping only upon success would risk > + * code elsewhere observing entry count being zero despite there actually > + * still being PoD entries. > + */ > + pod_lock(p2m); > + p2m->pod.entry_count += (1UL << order) - pod_count; > + pod_unlock(p2m); > + > /* Now, actually do the two-way mapping */ > rc = p2m_set_entry(p2m, gfn, INVALID_MFN, order, > p2m_populate_on_demand, p2m->default_access); > if ( rc == 0 ) > - { > - pod_lock(p2m); > - p2m->pod.entry_count += 1UL << order; > - p2m->pod.entry_count -= pod_count; > - BUG_ON(p2m->pod.entry_count < 0); > - pod_unlock(p2m); > - > ioreq_request_mapcache_invalidate(d); > - } > else if ( order ) > { > /* > @@ -1366,6 +1369,14 @@ mark_populate_on_demand(struct domain *d > d, gfn_l, order, rc); > domain_crash(d); > } > + else if ( !pod_count ) > + { > + /* Undo earlier increment; see comment above. */ > + pod_lock(p2m); > + BUG_ON(!p2m->pod.entry_count); > + --p2m->pod.entry_count; > + pod_unlock(p2m); > + } > > out: > gfn_unlock(p2m, gfn, order); > >
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