[Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[madhuparnabhowmik04]

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)) {
 		/* 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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Re: [Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[Wei Liu]

Thanks for the patch.

There is a typo in the subject line. It should say xen-netback, not
xen-netbank.

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.

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.

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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Re: [Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[Madhuparna Bhowmik]

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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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Re: [Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[Wei Liu]

On Wed, Jan 15, 2020 at 07:36:38PM +0530, Madhuparna Bhowmik wrote:
[...]
> 
> > 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.

I think you meant CONFIG_PROVE_RCU_LIST.

> 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.

Fair enough.

Wei.

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Re: [Xen-devel] [PATCH] net: xen-netbank: hash.c: Use built-in RCU list checking

[Madhuparna Bhowmik]

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On Wed, Jan 15, 2020 at 8:34 PM Wei Liu <wei.liu@kernel.org> wrote:

> On Wed, Jan 15, 2020 at 07:36:38PM +0530, Madhuparna Bhowmik wrote:
> [...]
> >
> > > 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.
>
> I think you meant CONFIG_PROVE_RCU_LIST.
>
> I am sorry about this. Yes, I meant  CONFIG_PROVE_RCU_LIST.

> 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.
>
> Fair enough.
>
> Thank you,
Madhuparna


> Wei.
>

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