From: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
To: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>,
Joel Fernandes <joel@joelfernandes.org>,
Paolo Bonzini <pbonzini@redhat.com>,
sean.j.christopherson@intel.com, vkuznets@redhat.com,
wanpengli@tencent.com, jmattson@google.com, tglx@linutronix.de,
bp@alien8.de, x86@kernel.org, kvm@vger.kernel.org,
linux-kernel@vger.kernel.org, frextrite@gmail.com,
linux-kernel-mentees@lists.linuxfoundation.org
Subject: Re: [PATCH v2] kvm: Fix false positive RCU usage warning
Date: Tue, 23 Jun 2020 23:19:20 +0530 [thread overview]
Message-ID: <20200623174920.GA13794@madhuparna-HP-Notebook> (raw)
In-Reply-To: <20200623153901.GG9247@paulmck-ThinkPad-P72>
On Tue, Jun 23, 2020 at 08:39:01AM -0700, Paul E. McKenney wrote:
> On Tue, Jun 23, 2020 at 09:00:36PM +0530, Madhuparna Bhowmik wrote:
> > On Tue, Jun 23, 2020 at 11:02:36AM -0400, Joel Fernandes wrote:
> > > 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.
> > >
> > Can we really get rid of the last argument? We would need the
> > srcu_struct right for checking?
>
> Agreed! However, the API could be simplified by passing in a pointer to
> the srcu_struct instead of a lockdep expression. An optional lockdep
> expression might still be helpful for calls from the update side,
> of course.
>
Sure, I will work on this.
Thanks,
Madhuparna
> Thanx, Paul
next prev parent reply other threads:[~2020-06-23 17:49 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-16 8:22 [PATCH v2] kvm: Fix false positive RCU usage warning madhuparnabhowmik10
2020-06-23 7:39 ` Paolo Bonzini
2020-06-23 15:02 ` Joel Fernandes
2020-06-23 15:30 ` Madhuparna Bhowmik
2020-06-23 15:39 ` Paul E. McKenney
2020-06-23 15:43 ` Joel Fernandes
2020-06-23 17:49 ` Madhuparna Bhowmik [this message]
2020-06-23 19:34 ` Joel Fernandes
2020-06-23 15:29 ` Madhuparna Bhowmik
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