Messages in this thread Patch in this message | | | From | John Ogness <> | Subject | [PATCH printk v5 12/40] console: introduce wrappers to read/write console flags | Date | Wed, 16 Nov 2022 17:27:24 +0106 |
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After switching to SRCU for console list iteration, some readers will begin readings console->flags as a data race. Locklessly reading console->flags provides a consistent value because there is at most one CPU modifying console->flags and that CPU is using only read-modify-write operations.
Introduce a wrapper for SRCU iterators to read console flags. Introduce a matching wrapper to write to flags of registered consoles. Writing to flags of registered consoles is synchronized by the console_list_lock.
Signed-off-by: John Ogness <john.ogness@linutronix.de> Reviewed-by: Petr Mladek <pmladek@suse.com> --- include/linux/console.h | 45 +++++++++++++++++++++++++++++++++++++++++ kernel/printk/printk.c | 10 ++++----- 2 files changed, 50 insertions(+), 5 deletions(-)
diff --git a/include/linux/console.h b/include/linux/console.h index 24d83e24840b..c1ca461d088a 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -183,6 +183,51 @@ extern void console_list_unlock(void) __releases(console_mutex); extern struct hlist_head console_list; +/** + * console_srcu_read_flags - Locklessly read the console flags + * @con: struct console pointer of console to read flags from + * + * This function provides the necessary READ_ONCE() and data_race() + * notation for locklessly reading the console flags. The READ_ONCE() + * in this function matches the WRITE_ONCE() when @flags are modified + * for registered consoles with console_srcu_write_flags(). + * + * Only use this function to read console flags when locklessly + * iterating the console list via srcu. + * + * Context: Any context. + */ +static inline short console_srcu_read_flags(const struct console *con) +{ + WARN_ON_ONCE(!console_srcu_read_lock_is_held()); + + /* + * Locklessly reading console->flags provides a consistent + * read value because there is at most one CPU modifying + * console->flags and that CPU is using only read-modify-write + * operations to do so. + */ + return data_race(READ_ONCE(con->flags)); +} + +/** + * console_srcu_write_flags - Write flags for a registered console + * @con: struct console pointer of console to write flags to + * @flags: new flags value to write + * + * Only use this function to write flags for registered consoles. It + * requires holding the console_list_lock. + * + * Context: Any context. + */ +static inline void console_srcu_write_flags(struct console *con, short flags) +{ + lockdep_assert_console_list_lock_held(); + + /* This matches the READ_ONCE() in console_srcu_read_flags(). */ + WRITE_ONCE(con->flags, flags); +} + /** * for_each_console_srcu() - Iterator over registered consoles * @con: struct console pointer used as loop cursor diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index f7479fd73114..35018f18f5aa 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3068,7 +3068,7 @@ void console_stop(struct console *console) __pr_flush(console, 1000, true); console_list_lock(); console_lock(); - console->flags &= ~CON_ENABLED; + console_srcu_write_flags(console, console->flags & ~CON_ENABLED); console_unlock(); console_list_unlock(); @@ -3086,7 +3086,7 @@ void console_start(struct console *console) { console_list_lock(); console_lock(); - console->flags |= CON_ENABLED; + console_srcu_write_flags(console, console->flags | CON_ENABLED); console_unlock(); console_list_unlock(); __pr_flush(console, 1000, true); @@ -3342,7 +3342,7 @@ void register_console(struct console *newcon) } else if (newcon->flags & CON_CONSDEV) { /* Only the new head can have CON_CONSDEV set. */ - console_first()->flags &= ~CON_CONSDEV; + console_srcu_write_flags(console_first(), console_first()->flags & ~CON_CONSDEV); hlist_add_head_rcu(&newcon->node, &console_list); } else { @@ -3399,7 +3399,7 @@ static int unregister_console_locked(struct console *console) console_lock(); /* Disable it unconditionally */ - console->flags &= ~CON_ENABLED; + console_srcu_write_flags(console, console->flags & ~CON_ENABLED); if (hlist_unhashed(&console->node)) { console_unlock(); @@ -3418,7 +3418,7 @@ static int unregister_console_locked(struct console *console) * console has any device attached. Oh well.... */ if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV) - console_first()->flags |= CON_CONSDEV; + console_srcu_write_flags(console_first(), console_first()->flags | CON_CONSDEV); console_unlock(); -- 2.30.2
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