From: Madhuparna Bhowmik <[email protected]>
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 <[email protected]>
Signed-off-by: Madhuparna Bhowmik <[email protected]>
---
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);
--
2.17.1
On 16/05/20 10:22, [email protected] wrote:
> From: Madhuparna Bhowmik <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Madhuparna Bhowmik <[email protected]>
> ---
> 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.
Paul, do you have any objections to the idea? Thanks,
Paolo
On Tue, Jun 23, 2020 at 09:39:53AM +0200, Paolo Bonzini wrote:
> On 16/05/20 10:22, [email protected] wrote:
> > From: Madhuparna Bhowmik <[email protected]>
> >
> > 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 <[email protected]>
> > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > ---
> > 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.
On Tue, Jun 23, 2020 at 09:39:53AM +0200, Paolo Bonzini wrote:
> On 16/05/20 10:22, [email protected] wrote:
> > From: Madhuparna Bhowmik <[email protected]>
> >
> > 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 <[email protected]>
> > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > ---
> > 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
>
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, [email protected] wrote:
> > > From: Madhuparna Bhowmik <[email protected]>
> > >
> > > 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 <[email protected]>
> > > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > > ---
> > > 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?
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, [email protected] wrote:
> > > > From: Madhuparna Bhowmik <[email protected]>
> > > >
> > > > 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 <[email protected]>
> > > > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > > > ---
> > > > 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
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, [email protected] wrote:
> > > > > From: Madhuparna Bhowmik <[email protected]>
> > > > >
> > > > > 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 <[email protected]>
> > > > > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > > > > ---
> > > > > 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.
That's true!
thanks,
- Joel
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, [email protected] wrote:
> > > > > From: Madhuparna Bhowmik <[email protected]>
> > > > >
> > > > > 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 <[email protected]>
> > > > > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > > > > ---
> > > > > 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
On Tue, Jun 23, 2020 at 11:19:20PM +0530, Madhuparna Bhowmik wrote:
> 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, [email protected] wrote:
> > > > > > From: Madhuparna Bhowmik <[email protected]>
> > > > > >
> > > > > > 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 <[email protected]>
> > > > > > Signed-off-by: Madhuparna Bhowmik <[email protected]>
> > > > > > ---
> > > > > > 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.
Cool, thanks !!!
- Joel