Hi Greg KH: On Fri, 1 Jan 2021 09:18:48 +0100, Greg KH wrote: >On Fri, Jan 01, 2021 at 11:37:18AM +0800, Gao Yan wrote: >> In tty layer, it provides tty->ldisc_sem to protect all tty_ldisc_ops >> including ppp_sync_ldisc. So I think tty->ldisc_sem can also protect >> tty->disc_data, and the disc_data_lock is not necessary. >> >> Signed-off-by: Gao Yan <gao.yanB@xxxxxxx> >> --- >> drivers/net/ppp/ppp_async.c | 11 ++--------- >> drivers/net/ppp/ppp_synctty.c | 12 ++---------- >> 2 files changed, 4 insertions(+), 19 deletions(-) > >What changed from v1? just change some description. >And how did you test this? Why remove this lock, is it causing problems somewhere for it to be here? Somedays ago, There is a problem of 4.14 kernel in n_tty line discipline. Specific description is here: Link: https://lkml.org/lkml/2020/12/9/339 At the beginning I tried to add a lock in the layer of n_tty, until I find the patch that helps me a lot. Link: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?h=v5.9-rc4&id=83d817f41070c48bc3eb7ec18e43000a548fca5c About the patch, Specific description is here: Link: https://lkml.org/lkml/2018/8/29/555 So after referring to the previous patch, it is unnecessary to add a lock to protect disc_data in n_tty_close and n_tty_flush_buffer. And I think it is the same with ppp line discipline. More detailed explanation: We have a potential race on dereferencing tty->disc_data, so we should use some locks to avoid the competition. In the current version, it defines disc_data_lock to protect the race of ppp_asynctty_receive and ppp_asynctty_close. However, I think when cpu A is running ppp_asynctty_receive, another cpu B will not run ppp_asynctty_close at the same time. The reasons are as follows: Cpu A will hold tty->ldisc_sem before running ppp_asynctty_receive. If cpu B wants to run ppp_asynctty_close, it must wait until cpu A release tty->ldisc_sem after ppp_asynctty_receive. So I think tty->ldisc_sem already can protect the tty->disc_data in ppp line discipline just like n_tty line discipline. Thanks. Gao Yan >Signed-off-by: Gao Yan <gao.yanB@xxxxxxx> >--- > 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 >
