[PATCH] net: remove disc_data_lock in ppp line discipline

[PATCH] net: remove disc_data_lock in ppp line discipline

[Gao Yan]

In tty layer, it use tty->ldisc_sem to proect tty_ldisc_ops.
So I think tty->ldisc_sem can also protect tty->disc_data;
For examlpe,
When cpu A is running ppp_synctty_ioctl that hold the tty->ldisc_sem,
at the same time  if cpu B calls ppp_synctty_close, it will wait until
cpu A release tty->ldisc_sem. So I think it is unnecessary to have the
disc_data_lock;

cpu A                           cpu B
tty_ioctl                       tty_reopen
 ->hold tty->ldisc_sem            ->hold tty->ldisc_sem(write), failed
 ->ld->ops->ioctl                 ->wait...
 ->release tty->ldisc_sem         ->wait...OK,hold tty->ldisc_sem
                                    ->tty_ldisc_reinit
                                      ->tty_ldisc_close
                                        ->ld->ops->close

Signed-off-by: Gao Yan <gao.yanB@h3c.com>
---
 drivers/net/ppp/ppp_async.c   | 11 ++---------
 drivers/net/ppp/ppp_synctty.c | 12 ++----------
 2 files changed, 4 insertions(+), 19 deletions(-)

diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
index 29a0917a8..20b50facd 100644
--- a/drivers/net/ppp/ppp_async.c
+++ b/drivers/net/ppp/ppp_async.c
@@ -127,17 +127,13 @@ static const struct ppp_channel_ops async_ops = {
  * FIXME: this is no longer true. The _close path for the ldisc is
  * now guaranteed to be sane.
  */
-static DEFINE_RWLOCK(disc_data_lock);
 
 static struct asyncppp *ap_get(struct tty_struct *tty)
 {
-	struct asyncppp *ap;
+	struct asyncppp *ap = tty->disc_data;
 
-	read_lock(&disc_data_lock);
-	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -214,12 +210,9 @@ ppp_asynctty_open(struct tty_struct *tty)
 static void
 ppp_asynctty_close(struct tty_struct *tty)
 {
-	struct asyncppp *ap;
+	struct asyncppp *ap = tty->disc_data;
 
-	write_lock_irq(&disc_data_lock);
-	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 0f338752c..53fb68e29 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -129,17 +129,12 @@ ppp_print_buffer (const char *name, const __u8 *buf, int count)
  *
  * FIXME: Fixed in tty_io nowadays.
  */
-static DEFINE_RWLOCK(disc_data_lock);
-
 static struct syncppp *sp_get(struct tty_struct *tty)
 {
-	struct syncppp *ap;
+	struct syncppp *ap = tty->disc_data;
 
-	read_lock(&disc_data_lock);
-	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -213,12 +208,9 @@ ppp_sync_open(struct tty_struct *tty)
 static void
 ppp_sync_close(struct tty_struct *tty)
 {
-	struct syncppp *ap;
+	struct syncppp *ap = tty->disc_data;
 
-	write_lock_irq(&disc_data_lock);
-	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
-- 
2.17.1

答复: [PATCH] net: remove disc_data_lock in ppp line discipline

[Gaoyan]

Dear all：

Could I get your comments for the updates?  If I can get a reply, it will help me a lot . Thanks

https://lkml.org/lkml/2020/12/28/19



*******************************


----Original mail -----
发件人: gaoyan (RD) 
发送时间: 2020年12月28日 15:16
收件人: paulus@samba.org; davem@davemloft.net; kuba@kernel.org
抄送: linux-ppp@vger.kernel.org; netdev@vger.kernel.org; linux-kernel@vger.kernel.org; gaoyan (RD) <gao.yanB@h3c.com>
主题: [PATCH] net: remove disc_data_lock in ppp line discipline

In tty layer, it use tty->ldisc_sem to proect tty_ldisc_ops.
So I think tty->ldisc_sem can also protect tty->disc_data; For examlpe, When cpu A is running ppp_synctty_ioctl that hold the tty->ldisc_sem, at the same time  if cpu B calls ppp_synctty_close, it will wait until cpu A release tty->ldisc_sem. So I think it is unnecessary to have the disc_data_lock;

cpu A                           cpu B
tty_ioctl                       tty_reopen
 ->hold tty->ldisc_sem            ->hold tty->ldisc_sem(write), failed
 ->ld->ops->ioctl                 ->wait...
 ->release tty->ldisc_sem         ->wait...OK,hold tty->ldisc_sem
                                    ->tty_ldisc_reinit
                                      ->tty_ldisc_close
                                        ->ld->ops->close

Signed-off-by: Gao Yan <gao.yanB@h3c.com>
---
 drivers/net/ppp/ppp_async.c   | 11 ++---------
 drivers/net/ppp/ppp_synctty.c | 12 ++----------
 2 files changed, 4 insertions(+), 19 deletions(-)

diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c index 29a0917a8..20b50facd 100644
--- a/drivers/net/ppp/ppp_async.c
+++ b/drivers/net/ppp/ppp_async.c
@@ -127,17 +127,13 @@ static const struct ppp_channel_ops async_ops = {
  * FIXME: this is no longer true. The _close path for the ldisc is
  * now guaranteed to be sane.
  */
-static DEFINE_RWLOCK(disc_data_lock);
 
 static struct asyncppp *ap_get(struct tty_struct *tty)  {
-	struct asyncppp *ap;
+	struct asyncppp *ap = tty->disc_data;
 
-	read_lock(&disc_data_lock);
-	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -214,12 +210,9 @@ ppp_asynctty_open(struct tty_struct *tty)  static void  ppp_asynctty_close(struct tty_struct *tty)  {
-	struct asyncppp *ap;
+	struct asyncppp *ap = tty->disc_data;
 
-	write_lock_irq(&disc_data_lock);
-	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c index 0f338752c..53fb68e29 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -129,17 +129,12 @@ ppp_print_buffer (const char *name, const __u8 *buf, int count)
  *
  * FIXME: Fixed in tty_io nowadays.
  */
-static DEFINE_RWLOCK(disc_data_lock);
-
 static struct syncppp *sp_get(struct tty_struct *tty)  {
-	struct syncppp *ap;
+	struct syncppp *ap = tty->disc_data;
 
-	read_lock(&disc_data_lock);
-	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -213,12 +208,9 @@ ppp_sync_open(struct tty_struct *tty)  static void  ppp_sync_close(struct tty_struct *tty)  {
-	struct syncppp *ap;
+	struct syncppp *ap = tty->disc_data;
 
-	write_lock_irq(&disc_data_lock);
-	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
--
2.17.1

Re: [PATCH] net: remove disc_data_lock in ppp line discipline

[Xie He]

> In tty layer, it use tty->ldisc_sem to proect tty_ldisc_ops.
> So I think tty->ldisc_sem can also protect tty->disc_data;

It might help by CC'ing TTY people, so that we could get this reviewed by
people who are familiar with TTY code.

Greg Kroah-Hartman <gregkh@linuxfoundation.org> (supporter:TTY LAYER)
Jiri Slaby <jirislaby@kernel.org> (supporter:TTY LAYER)

Thanks!

> For examlpe,
> When cpu A is running ppp_synctty_ioctl that hold the tty->ldisc_sem,
> at the same time  if cpu B calls ppp_synctty_close, it will wait until
> cpu A release tty->ldisc_sem. So I think it is unnecessary to have the
> disc_data_lock;
> 
> cpu A                           cpu B
> tty_ioctl                       tty_reopen
>  ->hold tty->ldisc_sem            ->hold tty->ldisc_sem(write), failed
>  ->ld->ops->ioctl                 ->wait...
>  ->release tty->ldisc_sem         ->wait...OK,hold tty->ldisc_sem
>                                     ->tty_ldisc_reinit
>                                       ->tty_ldisc_close
>                                         ->ld->ops->close

IMHO an example might not be necessary. Examples are useful to show
incorrectness. But we cannot show correctness by examples because
examples are not exhaustive.

BTW, there're some typos:
"proect" -> "protect"
"examlpe" -> "example"
"that hold ..." -> "that holds ..."
"cpu A release ..." -> "cpu A releases ..."

>   * FIXME: this is no longer true. The _close path for the ldisc is
>   * now guaranteed to be sane.
>   */

>   *
>   * FIXME: Fixed in tty_io nowadays.
>   */

Since you are removing "disc_data_lock", please update (or remove) these
two comments. Thanks!

Re: [PATCH] net: remove disc_data_lock in ppp line discipline

[Jakub Kicinski]

On Tue, 15 Dec 2020 23:00:54 +0800 Gao Yan wrote:
> tty layer provide tty->ldisc_sem lock to protect tty->disc_data;
> For examlpe, when cpu A is running ppp_synctty_ioctl that
> hold the tty->ldisc_sem, so if cpu B calls ppp_synctty_close,
> it will wait until cpu A release tty->ldisc_sem. So I think it is
> unnecessary to have the disc_data_lock;
> 
> cpu A                           cpu B
> tty_ioctl                       tty_reopen
>  ->hold tty->ldisc_sem            ->hold tty->ldisc_sem(write), failed
>  ->ld->ops->ioctl                 ->wait...
>  ->release tty->ldisc_sem         ->wait...OK,hold tty->ldisc_sem
>                                     ->tty_ldisc_reinit
>                                       ->tty_ldisc_close
>                                         ->ld->ops->close  
> 
> Signed-off-by: Gao Yan <gao.yanB@h3c.com>

# Form letter - net-next is closed

We have already sent the networking pull request for 5.11 and therefore
net-next is closed for new drivers, features, code refactoring and
optimizations. We are currently accepting bug fixes only.

Please repost when net-next reopens after 5.11-rc1 is cut.

Look out for the announcement on the mailing list or check:
http://vger.kernel.org/~davem/net-next.html

RFC patches sent for review only are obviously welcome at any time.

[PATCH] net: remove disc_data_lock in ppp line discipline

[Gao Yan]

tty layer provide tty->ldisc_sem lock to protect tty->disc_data;
For examlpe, when cpu A is running ppp_synctty_ioctl that
hold the tty->ldisc_sem, so if cpu B calls ppp_synctty_close,
it will wait until cpu A release tty->ldisc_sem. So I think it is
unnecessary to have the disc_data_lock;

cpu A                           cpu B
tty_ioctl                       tty_reopen
 ->hold tty->ldisc_sem            ->hold tty->ldisc_sem(write), failed
 ->ld->ops->ioctl                 ->wait...
 ->release tty->ldisc_sem         ->wait...OK,hold tty->ldisc_sem
                                    ->tty_ldisc_reinit
                                      ->tty_ldisc_close
                                        ->ld->ops->close

Signed-off-by: Gao Yan <gao.yanB@h3c.com>
---
 drivers/net/ppp/ppp_async.c   | 5 -----
 drivers/net/ppp/ppp_synctty.c | 5 -----
 2 files changed, 10 deletions(-)

diff --git a/drivers/net/ppp/ppp_async.c b/drivers/net/ppp/ppp_async.c
index 29a0917a8..f8cb591d6 100644
--- a/drivers/net/ppp/ppp_async.c
+++ b/drivers/net/ppp/ppp_async.c
@@ -127,17 +127,14 @@ static const struct ppp_channel_ops async_ops = {
  * FIXME: this is no longer true. The _close path for the ldisc is
  * now guaranteed to be sane.
  */
-static DEFINE_RWLOCK(disc_data_lock);
 
 static struct asyncppp *ap_get(struct tty_struct *tty)
 {
 	struct asyncppp *ap;
 
-	read_lock(&disc_data_lock);
 	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -216,10 +213,8 @@ ppp_asynctty_close(struct tty_struct *tty)
 {
 	struct asyncppp *ap;
 
-	write_lock_irq(&disc_data_lock);
 	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
diff --git a/drivers/net/ppp/ppp_synctty.c b/drivers/net/ppp/ppp_synctty.c
index 0f338752c..8cdf7268c 100644
--- a/drivers/net/ppp/ppp_synctty.c
+++ b/drivers/net/ppp/ppp_synctty.c
@@ -129,17 +129,14 @@ ppp_print_buffer (const char *name, const __u8 *buf, int count)
  *
  * FIXME: Fixed in tty_io nowadays.
  */
-static DEFINE_RWLOCK(disc_data_lock);
 
 static struct syncppp *sp_get(struct tty_struct *tty)
 {
 	struct syncppp *ap;
 
-	read_lock(&disc_data_lock);
 	ap = tty->disc_data;
 	if (ap != NULL)
 		refcount_inc(&ap->refcnt);
-	read_unlock(&disc_data_lock);
 	return ap;
 }
 
@@ -215,10 +212,8 @@ ppp_sync_close(struct tty_struct *tty)
 {
 	struct syncppp *ap;
 
-	write_lock_irq(&disc_data_lock);
 	ap = tty->disc_data;
 	tty->disc_data = NULL;
-	write_unlock_irq(&disc_data_lock);
 	if (!ap)
 		return;
 
-- 
2.17.1