| Message ID | 20221114162932.141883-32-john.ogness@linutronix.de (mailing list archive) |
|---|---|
| State | Not Applicable |
| Headers | show |
| Series | reduce console_lock scope | expand |
Hi, After more detailed runtime testing I discovered that I didn't re-insert the console to the correct place in the list. More below... On 2022-11-14, John Ogness <john.ogness@linutronix.de> wrote: > diff --git a/include/linux/console.h b/include/linux/console.h > index f716e1dd9eaf..9cea254b34b8 100644 > --- a/include/linux/console.h > +++ b/include/linux/console.h > @@ -291,6 +291,7 @@ enum con_flush_mode { > }; > > extern int add_preferred_console(char *name, int idx, char *options); > +extern void console_force_preferred_locked(struct console *con); > extern void register_console(struct console *); > extern int unregister_console(struct console *); > extern void console_lock(void); > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index e770b1ede6c9..dff76c1cef80 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3461,6 +3462,48 @@ int unregister_console(struct console *console) > } > EXPORT_SYMBOL(unregister_console); > > +/** > + * console_force_preferred_locked - force a registered console preferred > + * @con: The registered console to force preferred. > + * > + * Must be called under console_list_lock(). > + */ > +void console_force_preferred_locked(struct console *con) > +{ > + struct console *cur_pref_con; > + > + if (!console_is_registered_locked(con)) > + return; > + > + cur_pref_con = console_first(); > + > + /* Already preferred? */ > + if (cur_pref_con == con) > + return; > + > + /* > + * Delete, but do not re-initialize the entry. This allows the console > + * to continue to appear registered (via any hlist_unhashed_lockless() > + * checks), even though it was briefly removed from the console list. > + */ > + hlist_del_rcu(&con->node); > + > + /* > + * Ensure that all SRCU list walks have completed so that the console > + * can be added to the beginning of the console list and its forward > + * list pointer can be re-initialized. > + */ > + synchronize_srcu(&console_srcu); > + > + con->flags |= CON_CONSDEV; > + WARN_ON(!con->device); > + > + /* Only the new head can have CON_CONSDEV set. */ > + console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); > + hlist_add_behind_rcu(&con->node, console_list.first); This is adding the console as the 2nd item. It should be the new head. The patch below fixes it. I have done careful runtime testing with this fixup. After the force_preferred, the console is the new head and sending data to /dev/console redirects to that console. It would be nice if we could fold this in. Sorry. John Ogness diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 8d635467882f..4b77586cf4cb 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3494,7 +3494,7 @@ void console_force_preferred_locked(struct console *con) /* Only the new head can have CON_CONSDEV set. */ console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); - hlist_add_behind_rcu(&con->node, console_list.first); + hlist_add_head_rcu(&con->node, &console_list); } EXPORT_SYMBOL(console_force_preferred_locked);
On Mon 2022-11-14 20:57:18, John Ogness wrote: > Hi, > > After more detailed runtime testing I discovered that I didn't re-insert > the console to the correct place in the list. More below... > > > --- a/kernel/printk/printk.c > > +++ b/kernel/printk/printk.c > > @@ -3461,6 +3462,48 @@ int unregister_console(struct console *console) > > } > > EXPORT_SYMBOL(unregister_console); > > > > +/** > > + * console_force_preferred_locked - force a registered console preferred > > + * @con: The registered console to force preferred. > > + * > > + * Must be called under console_list_lock(). > > + */ > > +void console_force_preferred_locked(struct console *con) > > +{ > > + struct console *cur_pref_con; > > + > > + if (!console_is_registered_locked(con)) > > + return; > > + > > + cur_pref_con = console_first(); > > + > > + /* Already preferred? */ > > + if (cur_pref_con == con) > > + return; > > + > > + /* > > + * Delete, but do not re-initialize the entry. This allows the console > > + * to continue to appear registered (via any hlist_unhashed_lockless() > > + * checks), even though it was briefly removed from the console list. > > + */ > > + hlist_del_rcu(&con->node); > > + > > + /* > > + * Ensure that all SRCU list walks have completed so that the console > > + * can be added to the beginning of the console list and its forward > > + * list pointer can be re-initialized. > > + */ > > + synchronize_srcu(&console_srcu); > > + > > + con->flags |= CON_CONSDEV; > > + WARN_ON(!con->device); > > + > > + /* Only the new head can have CON_CONSDEV set. */ > > + console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); > > + hlist_add_behind_rcu(&con->node, console_list.first); > > This is adding the console as the 2nd item. It should be the new > head. The patch below fixes it. > > I have done careful runtime testing with this fixup. After the > force_preferred, the console is the new head and sending data to > /dev/console redirects to that console. Great catch! > It would be nice if we could fold this in. Sorry. I have missed it as well :-/ > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 8d635467882f..4b77586cf4cb 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -3494,7 +3494,7 @@ void console_force_preferred_locked(struct console *con) > > /* Only the new head can have CON_CONSDEV set. */ > console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); > - hlist_add_behind_rcu(&con->node, console_list.first); > + hlist_add_head_rcu(&con->node, &console_list); > } > EXPORT_SYMBOL(console_force_preferred_locked); With this change: Reviewed-by: Petr Mladek <pmladek@suse.com> Best Regards, Petr
diff --git a/drivers/video/fbdev/xen-fbfront.c b/drivers/video/fbdev/xen-fbfront.c index 4d2694d904aa..8752d389e382 100644 --- a/drivers/video/fbdev/xen-fbfront.c +++ b/drivers/video/fbdev/xen-fbfront.c @@ -504,18 +504,14 @@ static void xenfb_make_preferred_console(void) if (console_set_on_cmdline) return; - console_lock(); + console_list_lock(); for_each_console(c) { if (!strcmp(c->name, "tty") && c->index == 0) break; } - console_unlock(); - if (c) { - unregister_console(c); - c->flags |= CON_CONSDEV; - c->flags &= ~CON_PRINTBUFFER; /* don't print again */ - register_console(c); - } + if (c) + console_force_preferred_locked(c); + console_list_unlock(); } static int xenfb_resume(struct xenbus_device *dev) diff --git a/include/linux/console.h b/include/linux/console.h index f716e1dd9eaf..9cea254b34b8 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -291,6 +291,7 @@ enum con_flush_mode { }; extern int add_preferred_console(char *name, int idx, char *options); +extern void console_force_preferred_locked(struct console *con); extern void register_console(struct console *); extern int unregister_console(struct console *); extern void console_lock(void); diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index e770b1ede6c9..dff76c1cef80 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -247,9 +247,10 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write, void console_list_lock(void) { /* - * In unregister_console(), synchronize_srcu() is called with the - * console_list_lock held. Therefore it is not allowed that the - * console_list_lock is taken with the srcu_lock held. + * In unregister_console() and console_force_preferred_locked(), + * synchronize_srcu() is called with the console_list_lock held. + * Therefore it is not allowed that the console_list_lock is taken + * with the srcu_lock held. * * Detecting if this context is really in the read-side critical * section is only possible if the appropriate debug options are @@ -3461,6 +3462,48 @@ int unregister_console(struct console *console) } EXPORT_SYMBOL(unregister_console); +/** + * console_force_preferred_locked - force a registered console preferred + * @con: The registered console to force preferred. + * + * Must be called under console_list_lock(). + */ +void console_force_preferred_locked(struct console *con) +{ + struct console *cur_pref_con; + + if (!console_is_registered_locked(con)) + return; + + cur_pref_con = console_first(); + + /* Already preferred? */ + if (cur_pref_con == con) + return; + + /* + * Delete, but do not re-initialize the entry. This allows the console + * to continue to appear registered (via any hlist_unhashed_lockless() + * checks), even though it was briefly removed from the console list. + */ + hlist_del_rcu(&con->node); + + /* + * Ensure that all SRCU list walks have completed so that the console + * can be added to the beginning of the console list and its forward + * list pointer can be re-initialized. + */ + synchronize_srcu(&console_srcu); + + con->flags |= CON_CONSDEV; + WARN_ON(!con->device); + + /* Only the new head can have CON_CONSDEV set. */ + console_srcu_write_flags(cur_pref_con, cur_pref_con->flags & ~CON_CONSDEV); + hlist_add_behind_rcu(&con->node, console_list.first); +} +EXPORT_SYMBOL(console_force_preferred_locked); + /* * Initialize the console device. This is called *early*, so * we can't necessarily depend on lots of kernel help here.
With commit 9e124fe16ff2("xen: Enable console tty by default in domU if it's not a dummy") a hack was implemented to make sure that the tty console remains the console behind the /dev/console device. The main problem with the hack is that, after getting the console pointer to the tty console, it is assumed the pointer is still valid after releasing the console_sem. This assumption is incorrect and unsafe. Make the hack safe by introducing a new function console_force_preferred_locked() and perform the full operation under the console_list_lock. Signed-off-by: John Ogness <john.ogness@linutronix.de> --- drivers/video/fbdev/xen-fbfront.c | 12 +++----- include/linux/console.h | 1 + kernel/printk/printk.c | 49 +++++++++++++++++++++++++++++-- 3 files changed, 51 insertions(+), 11 deletions(-)