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McKenney" To: Madhuparna Bhowmik Message-ID: <20200623153901.GG9247@paulmck-ThinkPad-P72> References: <20200516082227.22194-1-madhuparnabhowmik10@gmail.com> <9fff3c6b-1978-c647-16f7-563a1cdf62ff@redhat.com> <20200623150236.GD9005@google.com> <20200623153036.GB9914@madhuparna-HP-Notebook> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20200623153036.GB9914@madhuparna-HP-Notebook> User-Agent: Mutt/1.9.4 (2018-02-28) Cc: wanpengli@tencent.com, kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org, sean.j.christopherson@intel.com, bp@alien8.de, Joel Fernandes , Paolo Bonzini , vkuznets@redhat.com, linux-kernel-mentees@lists.linuxfoundation.org, tglx@linutronix.de, jmattson@google.com Subject: Re: [Linux-kernel-mentees] [PATCH v2] kvm: Fix false positive RCU usage warning X-BeenThere: linux-kernel-mentees@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: paulmck@kernel.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: linux-kernel-mentees-bounces@lists.linuxfoundation.org Sender: "Linux-kernel-mentees" 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 > > > > > > > > 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 > > > > Signed-off-by: Madhuparna Bhowmik > > > > --- > > > > 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. Thanx, Paul _______________________________________________ Linux-kernel-mentees mailing list Linux-kernel-mentees@lists.linuxfoundation.org https://lists.linuxfoundation.org/mailman/listinfo/linux-kernel-mentees