Messages in this thread Patch in this message | | | From | John Ogness <> | Subject | [PATCH printk v3 07/40] console: introduce console_is_enabled() wrapper | Date | Mon, 7 Nov 2022 15:22:05 +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.
The primary reason for readers to access console->flags is to check if the console is enabled. Introduce console_is_enabled() to mark such access as a data race.
Signed-off-by: John Ogness <john.ogness@linutronix.de> --- include/linux/console.h | 27 +++++++++++++++++++++++++++ kernel/printk/printk.c | 10 +++++----- 2 files changed, 32 insertions(+), 5 deletions(-)
diff --git a/include/linux/console.h b/include/linux/console.h index f4f0c9523835..d9c636011364 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -172,6 +172,33 @@ extern void console_srcu_read_unlock(int cookie); extern struct hlist_head console_list; +/** + * console_is_enabled - Locklessly check if the console is enabled + * @con: struct console pointer of console to check + * + * Unless the caller is explicitly synchronizing against the console + * register/unregister/stop/start functions, this function should be + * used instead of manually readings console->flags and testing for + * the CON_ENABLED bit. + * + * 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. + * + * Context: Any context. + */ +static inline bool console_is_enabled(const struct console *con) +{ + /* + * 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)) & CON_ENABLED); +} + /** * 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 8974523f3107..79811984da34 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -3021,7 +3021,7 @@ void console_stop(struct console *console) { __pr_flush(console, 1000, true); console_lock(); - console->flags &= ~CON_ENABLED; + WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED); console_unlock(); /* @@ -3037,7 +3037,7 @@ EXPORT_SYMBOL(console_stop); void console_start(struct console *console) { console_lock(); - console->flags |= CON_ENABLED; + WRITE_ONCE(console->flags, console->flags | CON_ENABLED); console_unlock(); __pr_flush(console, 1000, true); } @@ -3256,7 +3256,7 @@ 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; + WRITE_ONCE(console_first()->flags, console_first()->flags & ~CON_CONSDEV); hlist_add_head_rcu(&newcon->node, &console_list); } else { @@ -3308,7 +3308,7 @@ int unregister_console(struct console *console) console_lock(); /* Disable it unconditionally */ - console->flags &= ~CON_ENABLED; + WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED); if (hlist_unhashed(&console->node)) { console_unlock(); @@ -3327,7 +3327,7 @@ int unregister_console(struct console *console) * console has any device attached. Oh well.... */ if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV) - console_first()->flags |= CON_CONSDEV; + WRITE_ONCE(console_first()->flags, console_first()->flags | CON_CONSDEV); console_unlock(); -- 2.30.2
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