Re: [Linux-kernel-mentees] [PATCH v2] kvm: Fix false positive RCU usage warning

[Joel Fernandes <joel@joelfernandes.org>]

On Tue, Jun 23, 2020 at 09:39:53AM +0200, Paolo Bonzini wrote:
> On 16/05/20 10:22, madhuparnabhowmik10@gmail.com wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > 
> > 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@intel.com>
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
> > ---
> > 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.
> 
> I think instead you should add hlist_for_each_srcu and
> hlist_for_each_entry_srcu macro to include/linux/rculist.h.
> 
> 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.

You are right, this patch is wrong, we need a new SRCU list macro to do the
right thing which would also get rid of the last list argument.

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