Re: [PATCH printk v2 03/38] printk: Prepare for SRCU console list protection

Paul E. McKenney
2022-10-19T17:12:37Z

On Wed, Oct 19, 2022 at 05:01:25PM +0206, John Ogness wrote:
> Provide an NMI-safe SRCU protected variant to walk the console list.
>
> Note that all console fields are now set before adding the console
> to the list to avoid the console becoming visible by SCRU readers
> before being fully initialized.
>
> This is a preparatory change for a new console infrastructure which
> operates independent of the console BKL.
>
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

From an RCU viewpoint:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
> .clang-format | 1 +
> include/linux/console.h | 28 +++++++++++++++-
> kernel/printk/printk.c | 72 ++++++++++++++++++++++++++++++++---------
> 3 files changed, 85 insertions(+), 16 deletions(-)
>
> diff --git a/.clang-format b/.clang-format
> index 1247d54f9e49..04a675b56b57 100644
> --- a/.clang-format
> +++ b/.clang-format
> @@ -222,6 +222,7 @@ ForEachMacros:
> - 'for_each_component_dais'
> - 'for_each_component_dais_safe'
> - 'for_each_console'
> + - 'for_each_console_srcu'
> - 'for_each_cpu'
> - 'for_each_cpu_and'
> - 'for_each_cpu_not'
> diff --git a/include/linux/console.h b/include/linux/console.h
> index 7b5f21f9e469..cff86cc615f8 100644
> --- a/include/linux/console.h
> +++ b/include/linux/console.h
> @@ -15,7 +15,7 @@
> #define _LINUX_CONSOLE_H_ 1
>
> #include <linux/atomic.h>
> -#include <linux/list.h>
> +#include <linux/rculist.h>
> #include <linux/types.h>
>
> struct vc_data;
> @@ -158,8 +158,34 @@ struct console {
> struct hlist_node node;
> };
>
> +#ifdef CONFIG_LOCKDEP
> +extern bool console_srcu_read_lock_is_held(void);
> +#else
> +static inline bool console_srcu_read_lock_is_held(void)
> +{
> + return 1;
> +}
> +#endif
> +
> +extern int console_srcu_read_lock(void);
> +extern void console_srcu_read_unlock(int cookie);
> +
> extern struct hlist_head console_list;
>
> +/**
> + * for_each_console_srcu() - Iterator over registered consoles
> + * @con: struct console pointer used as loop cursor
> + *
> + * Although SRCU guarantees the console list will be consistent, the
> + * struct console fields may be updated by other CPUs while iterating.
> + *
> + * Requires console_srcu_read_lock to be held. Can be invoked from
> + * any context.
> + */
> +#define for_each_console_srcu(con) \
> + hlist_for_each_entry_srcu(con, &console_list, node, \
> + console_srcu_read_lock_is_held())
> +
> /*
> * for_each_console() allows you to iterate on each console
> */
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index 867becc40021..e8a56056cd50 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -85,6 +85,7 @@ EXPORT_SYMBOL(oops_in_progress);
> static DEFINE_SEMAPHORE(console_sem);
> HLIST_HEAD(console_list);
> EXPORT_SYMBOL_GPL(console_list);
> +DEFINE_STATIC_SRCU(console_srcu);
>
> /*
> * System may need to suppress printk message under certain
> @@ -102,6 +103,11 @@ static int __read_mostly suppress_panic_printk;
> static struct lockdep_map console_lock_dep_map = {
> .name = "console_lock"
> };
> +
> +bool console_srcu_read_lock_is_held(void)
> +{
> + return srcu_read_lock_held(&console_srcu);
> +}
> #endif
>
> enum devkmsg_log_bits {
> @@ -219,6 +225,32 @@ int devkmsg_sysctl_set_loglvl(struct ctl_table *table, int write,
> }
> #endif /* CONFIG_PRINTK && CONFIG_SYSCTL */
>
> +/**
> + * console_srcu_read_lock - Register a new reader for the
> + * SRCU-protected console list
> + *
> + * Use for_each_console_srcu() to iterate the console list
> + *
> + * Context: Any context.
> + */
> +int console_srcu_read_lock(void)
> +{
> + return srcu_read_lock_nmisafe(&console_srcu);
> +}
> +EXPORT_SYMBOL(console_srcu_read_lock);
> +
> +/**
> + * console_srcu_read_unlock - Unregister an old reader from
> + * the SRCU-protected console list
> + *
> + * Counterpart to console_srcu_read_lock()
> + */
> +void console_srcu_read_unlock(int cookie)
> +{
> + srcu_read_unlock_nmisafe(&console_srcu, cookie);
> +}
> +EXPORT_SYMBOL(console_srcu_read_unlock);
> +
> /*
> * Helper macros to handle lockdep when locking/unlocking console_sem. We use
> * macros instead of functions so that _RET_IP_ contains useful information.
> @@ -2989,6 +3021,9 @@ void console_stop(struct console *console)
> console_lock();
> console->flags &= ~CON_ENABLED;
> console_unlock();
> +
> + /* Ensure that all SRCU list walks have completed */
> + synchronize_srcu(&console_srcu);
> }
> EXPORT_SYMBOL(console_stop);
>
> @@ -3179,6 +3214,17 @@ void register_console(struct console *newcon)
> newcon->flags &= ~CON_PRINTBUFFER;
> }
>
> + newcon->dropped = 0;
> + if (newcon->flags & CON_PRINTBUFFER) {
> + /* Get a consistent copy of @syslog_seq. */
> + mutex_lock(&syslog_lock);
> + newcon->seq = syslog_seq;
> + mutex_unlock(&syslog_lock);
> + } else {
> + /* Begin with next message. */
> + newcon->seq = prb_next_seq(prb);
> + }
> +
> /*
> * Put this console in the list - keep the
> * preferred driver at the head of the list.
> @@ -3187,28 +3233,20 @@ void register_console(struct console *newcon)
> if (hlist_empty(&console_list)) {
> /* Ensure CON_CONSDEV is always set for the head. */
> newcon->flags |= CON_CONSDEV;
> - hlist_add_head(&newcon->node, &console_list);
> + hlist_add_head_rcu(&newcon->node, &console_list);
>
> } else if (newcon->flags & CON_CONSDEV) {
> /* Only the new head can have CON_CONSDEV set. */
> console_first()->flags &= ~CON_CONSDEV;
> - hlist_add_head(&newcon->node, &console_list);
> + hlist_add_head_rcu(&newcon->node, &console_list);
>
> } else {
> - hlist_add_behind(&newcon->node, console_list.first);
> - }
> -
> - newcon->dropped = 0;
> - if (newcon->flags & CON_PRINTBUFFER) {
> - /* Get a consistent copy of @syslog_seq. */
> - mutex_lock(&syslog_lock);
> - newcon->seq = syslog_seq;
> - mutex_unlock(&syslog_lock);
> - } else {
> - /* Begin with next message. */
> - newcon->seq = prb_next_seq(prb);
> + hlist_add_behind_rcu(&newcon->node, console_list.first);
> }
> console_unlock();
> +
> + /* No need to synchronize SRCU here! */
> +
> console_sysfs_notify();
>
> /*
> @@ -3254,7 +3292,7 @@ int unregister_console(struct console *console)
> goto out_unlock;
> }
>
> - hlist_del_init(&console->node);
> + hlist_del_init_rcu(&console->node);
>
> /*
> * <HISTORICAL>
> @@ -3269,6 +3307,10 @@ int unregister_console(struct console *console)
> console_first()->flags |= CON_CONSDEV;
>
> console_unlock();
> +
> + /* Ensure that all SRCU list walks have completed */
> + synchronize_srcu(&console_srcu);
> +
> console_sysfs_notify();
>
> if (console->exit)
> --
> 2.30.2
>