On Tue, Jun 23, 2020 at 09:39:53AM +0200, Paolo Bonzini wrote: > On 16/05/20 10:22, madhuparnabhowmik10@xxxxxxxxx wrote: > > From: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx> > > > > Fix the following false positive warnings: > > > > [ 9403.765413][T61744] ============================= > > [ 9403.786541][T61744] WARNING: suspicious RCU usage > > [ 9403.807865][T61744] 5.7.0-rc1-next-20200417 #4 Tainted: G L > > [ 9403.838945][T61744] ----------------------------- > > [ 9403.860099][T61744] arch/x86/kvm/mmu/page_track.c:257 RCU-list traversed in non-reader section!! > > > > and > > > > [ 9405.859252][T61751] ============================= > > [ 9405.859258][T61751] WARNING: suspicious RCU usage > > [ 9405.880867][T61755] ----------------------------- > > [ 9405.911936][T61751] 5.7.0-rc1-next-20200417 #4 Tainted: G L > > [ 9405.911942][T61751] ----------------------------- > > [ 9405.911950][T61751] arch/x86/kvm/mmu/page_track.c:232 RCU-list traversed in non-reader section!! > > > > Since srcu read lock is held, these are false positive warnings. > > Therefore, pass condition srcu_read_lock_held() to > > list_for_each_entry_rcu(). > > > > Reported-by: kernel test robot <lkp@xxxxxxxxx> > > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@xxxxxxxxx> > > --- > > v2: > > -Rebase v5.7-rc5 > > > > arch/x86/kvm/mmu/page_track.c | 6 ++++-- > > 1 file changed, 4 insertions(+), 2 deletions(-) > > > > diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c > > index ddc1ec3bdacd..1ad79c7aa05b 100644 > > --- a/arch/x86/kvm/mmu/page_track.c > > +++ b/arch/x86/kvm/mmu/page_track.c > > @@ -229,7 +229,8 @@ void kvm_page_track_write(struct kvm_vcpu *vcpu, gpa_t gpa, const u8 *new, > > return; > > > > idx = srcu_read_lock(&head->track_srcu); > > - hlist_for_each_entry_rcu(n, &head->track_notifier_list, node) > > + hlist_for_each_entry_rcu(n, &head->track_notifier_list, node, > > + srcu_read_lock_held(&head->track_srcu)) > > if (n->track_write) > > n->track_write(vcpu, gpa, new, bytes, n); > > srcu_read_unlock(&head->track_srcu, idx); > > @@ -254,7 +255,8 @@ void kvm_page_track_flush_slot(struct kvm *kvm, struct kvm_memory_slot *slot) > > return; > > > > idx = srcu_read_lock(&head->track_srcu); > > - hlist_for_each_entry_rcu(n, &head->track_notifier_list, node) > > + hlist_for_each_entry_rcu(n, &head->track_notifier_list, node, > > + srcu_read_lock_held(&head->track_srcu)) > > if (n->track_flush_slot) > > n->track_flush_slot(kvm, slot, n); > > srcu_read_unlock(&head->track_srcu, idx); > > > > Hi, sorry for the delay in reviewing this patch. I would like to ask > Paul about it. > > While you're correctly fixing a false positive, hlist_for_each_entry_rcu > would have a false _negative_ if you called it under > rcu_read_lock/unlock and the data structure was protected by SRCU. This > is why for example srcu_dereference is used instead of > rcu_dereference_check, and why srcu_dereference uses > __rcu_dereference_check (with the two underscores) instead of > rcu_dereference_check. Using rcu_dereference_check would add an "|| > rcu_read_lock_held()" to the condition which is wrong. > Yes, that makes sense, there would be a false negative, thank you for pointing out this issue. > I think instead you should add hlist_for_each_srcu and > hlist_for_each_entry_srcu macro to include/linux/rculist.h. > This seems good to me, I can work on this, but I would wait for Paul's suggestion on this. > There is no need for equivalents of hlist_for_each_entry_continue_rcu > and hlist_for_each_entry_from_rcu, because they use rcu_dereference_raw. > However, it's not documented why they do so. > > Paul, do you have any objections to the idea? Thanks, > > Paolo Thank you, Madhuparna >
