lkml.org 
[lkml]   [2020]   [Jun]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
SubjectRe: [PATCH v2] kvm: Fix false positive RCU usage warning
From
Date
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.

Paul, do you have any objections to the idea? Thanks,

Paolo

\
 
 \ /
  Last update: 2020-06-23 09:40    [W:0.098 / U:0.068 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site