Re: [Linux-kernel-mentees] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

From: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
To: Wei Liu <wei.liu@kernel.org>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	paul@xen.org, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	Joel Fernandes <joel@joelfernandes.org>,
	xen-devel@lists.xenproject.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	davem@davemloft.net
Subject: Re: [Linux-kernel-mentees] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking
Date: Wed, 15 Jan 2020 19:36:38 +0530	[thread overview]
Message-ID: <CAF65HP0q_KcrUP_50JxZL1xNc47=detHvdOzjBmuiqUtB3AwfA@mail.gmail.com> (raw)
In-Reply-To: <20200115135631.edr2nrfkycppxcku@debian>

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On Wed, Jan 15, 2020 at 7:26 PM Wei Liu <wei.liu@kernel.org> wrote:

> Thanks for the patch.
>
> There is a typo in the subject line. It should say xen-netback, not
> xen-netbank.
>
> Hi,

I am sorry about this, I will send this patch again.

> On Wed, Jan 15, 2020 at 06:11:28PM +0530, madhuparnabhowmik04@gmail.com
> wrote:
> > From: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
> >
> > list_for_each_entry_rcu has built-in RCU and lock checking.
> > Pass cond argument to list_for_each_entry_rcu.
> >
> > Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
> > ---
> >  drivers/net/xen-netback/hash.c | 6 ++++--
> >  1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/net/xen-netback/hash.c
> b/drivers/net/xen-netback/hash.c
> > index 10d580c3dea3..30709bc9d170 100644
> > --- a/drivers/net/xen-netback/hash.c
> > +++ b/drivers/net/xen-netback/hash.c
> > @@ -51,7 +51,8 @@ static void xenvif_add_hash(struct xenvif *vif, const
> u8 *tag,
> >
> >       found = false;
> >       oldest = NULL;
> > -     list_for_each_entry_rcu(entry, &vif->hash.cache.list, link) {
> > +     list_for_each_entry_rcu(entry, &vif->hash.cache.list, link,
> > +
>  lockdep_is_held(&vif->hash.cache.lock)) {
>
> There are probably too many tabs here. Indentation looks wrong.
>
> I will correct this when I resend this patch.

> The surrounding code makes it pretty clear that the lock is already held
> by the time list_for_each_entry_rcu is called, yet the checking involved
> in lockdep_is_held is not trivial, so I'm afraid I don't consider this a
> strict improvement over the existing code.
>
> Actually,  we want to make CONFIG_PROVE_LIST_RCU enabled by default.
And if the cond argument is not passed when the usage of
list_for_each_entry_rcu()
is outside of rcu_read_lock(), it will lead to a false positive.
Therefore, I think this patch is required.
Let me know if you have any objections.

Thank you,
Madhuparna

> If there is something I misunderstood, let me know.
>
> Wei.
>
> >               /* Make sure we don't add duplicate entries */
> >               if (entry->len == len &&
> >                   memcmp(entry->tag, tag, len) == 0)
> > @@ -102,7 +103,8 @@ static void xenvif_flush_hash(struct xenvif *vif)
> >
> >       spin_lock_irqsave(&vif->hash.cache.lock, flags);
> >
> > -     list_for_each_entry_rcu(entry, &vif->hash.cache.list, link) {
> > +     list_for_each_entry_rcu(entry, &vif->hash.cache.list, link,
> > +
>  lockdep_is_held(&vif->hash.cache.lock)) {
> >               list_del_rcu(&entry->link);
> >               vif->hash.cache.count--;
> >               kfree_rcu(entry, rcu);
> > --
> > 2.17.1
> >
>

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