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

["Paul E. McKenney" <paulmck@kernel.org>]

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
>