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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
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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<div dir="ltr"><div dir="ltr"><br></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Wed, Jan 15, 2020 at 7:26 PM Wei Liu &lt;<a href="mailto:wei.liu@kernel.org">wei.liu@kernel.org</a>&gt; wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Thanks for the patch.<br>
<br>
There is a typo in the subject line. It should say xen-netback, not<br>
xen-netbank.<br>
<br></blockquote><div>Hi,</div><div><br></div><div>I am sorry about this, I will send this patch again.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
On Wed, Jan 15, 2020 at 06:11:28PM +0530, <a href="mailto:madhuparnabhowmik04@gmail.com" target="_blank">madhuparnabhowmik04@gmail.com</a> wrote:<br>
&gt; From: Madhuparna Bhowmik &lt;<a href="mailto:madhuparnabhowmik04@gmail.com" target="_blank">madhuparnabhowmik04@gmail.com</a>&gt;<br>
&gt; <br>
&gt; list_for_each_entry_rcu has built-in RCU and lock checking.<br>
&gt; Pass cond argument to list_for_each_entry_rcu.<br>
&gt; <br>
&gt; Signed-off-by: Madhuparna Bhowmik &lt;<a href="mailto:madhuparnabhowmik04@gmail.com" target="_blank">madhuparnabhowmik04@gmail.com</a>&gt;<br>
&gt; ---<br>
&gt;  drivers/net/xen-netback/hash.c | 6 ++++--<br>
&gt;  1 file changed, 4 insertions(+), 2 deletions(-)<br>
&gt; <br>
&gt; diff --git a/drivers/net/xen-netback/hash.c b/drivers/net/xen-netback/hash.c<br>
&gt; index 10d580c3dea3..30709bc9d170 100644<br>
&gt; --- a/drivers/net/xen-netback/hash.c<br>
&gt; +++ b/drivers/net/xen-netback/hash.c<br>
&gt; @@ -51,7 +51,8 @@ static void xenvif_add_hash(struct xenvif *vif, const u8 *tag,<br>
&gt;  <br>
&gt;       found = false;<br>
&gt;       oldest = NULL;<br>
&gt; -     list_for_each_entry_rcu(entry, &amp;vif-&gt;hash.cache.list, link) {<br>
&gt; +     list_for_each_entry_rcu(entry, &amp;vif-&gt;hash.cache.list, link,<br>
&gt; +                                                     lockdep_is_held(&amp;vif-&gt;hash.cache.lock)) {<br>
<br>
There are probably too many tabs here. Indentation looks wrong.<br>
<br></blockquote><div>I will correct this when I resend this patch.</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
The surrounding code makes it pretty clear that the lock is already held<br>
by the time list_for_each_entry_rcu is called, yet the checking involved<br>
in lockdep_is_held is not trivial, so I&#39;m afraid I don&#39;t consider this a<br>
strict improvement over the existing code.<br>
<br></blockquote><div>Actually,  we want to make CONFIG_PROVE_LIST_RCU enabled by default.</div><div>And if the cond argument is not passed when the usage of list_for_each_entry_rcu()</div><div>is outside of rcu_read_lock(), it will lead to a false positive.</div><div>Therefore, I think this patch is required.</div><div>Let me know if you have any objections.</div><div><br></div><div>Thank you,</div><div>Madhuparna</div><div> </div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
If there is something I misunderstood, let me know.<br>
<br>
Wei.<br>
<br>
&gt;               /* Make sure we don&#39;t add duplicate entries */<br>
&gt;               if (entry-&gt;len == len &amp;&amp;<br>
&gt;                   memcmp(entry-&gt;tag, tag, len) == 0)<br>
&gt; @@ -102,7 +103,8 @@ static void xenvif_flush_hash(struct xenvif *vif)<br>
&gt;  <br>
&gt;       spin_lock_irqsave(&amp;vif-&gt;hash.cache.lock, flags);<br>
&gt;  <br>
&gt; -     list_for_each_entry_rcu(entry, &amp;vif-&gt;hash.cache.list, link) {<br>
&gt; +     list_for_each_entry_rcu(entry, &amp;vif-&gt;hash.cache.list, link,<br>
&gt; +                                                     lockdep_is_held(&amp;vif-&gt;hash.cache.lock)) {<br>
&gt;               list_del_rcu(&amp;entry-&gt;link);<br>
&gt;               vif-&gt;hash.cache.count--;<br>
&gt;               kfree_rcu(entry, rcu);<br>
&gt; -- <br>
&gt; 2.17.1<br>
&gt; <br>
</blockquote></div></div>

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Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-15 12:41 madhuparnabhowmik04
2020-01-15 13:56 ` Wei Liu
2020-01-15 14:06   ` Madhuparna Bhowmik [this message]
2020-01-15 15:04     ` Wei Liu
2020-01-15 15:46       ` Madhuparna Bhowmik

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