From: John Ogness Date: Tue, 20 Aug 2024 08:35:39 +0206 Subject: [PATCH 13/54] nbcon: Add API to acquire context for non-printing operations Origin: https://www.kernel.org/pub/linux/kernel/projects/rt/6.11/older/patches-6.11-rt7.tar.xz Provide functions nbcon_device_try_acquire() and nbcon_device_release() which will try to acquire the nbcon console ownership with NBCON_PRIO_NORMAL and mark it unsafe for handover/takeover. These functions are to be used together with the device-specific locking when performing non-printing activities on the console device. They will allow synchronization against the atomic_write() callback which will be serialized, for higher priority contexts, only by acquiring the console context ownership. Pitfalls: The API requires to be called in a context with migration disabled because it uses per-CPU variables internally. The context is set unsafe for a takeover all the time. It guarantees full serialization against any atomic_write() caller except for the final flush in panic() which might try an unsafe takeover. Signed-off-by: John Ogness Reviewed-by: Petr Mladek Link: https://lore.kernel.org/r/20240820063001.36405-14-john.ogness@linutronix.de Signed-off-by: Petr Mladek Signed-off-by: Sebastian Andrzej Siewior --- include/linux/console.h | 2 + include/linux/printk.h | 14 +++++++++++ kernel/printk/nbcon.c | 58 +++++++++++++++++++++++++++++++++++++++++++++++- 3 files changed, 73 insertions(+), 1 deletion(-) --- a/include/linux/console.h +++ b/include/linux/console.h @@ -322,6 +322,7 @@ struct nbcon_write_context { * * @nbcon_state: State for nbcon consoles * @nbcon_seq: Sequence number of the next record for nbcon to print + * @nbcon_device_ctxt: Context available for non-printing operations * @pbufs: Pointer to nbcon private buffer */ struct console { @@ -417,6 +418,7 @@ struct console { atomic_t __private nbcon_state; atomic_long_t __private nbcon_seq; + struct nbcon_context __private nbcon_device_ctxt; struct printk_buffers *pbufs; }; --- a/include/linux/printk.h +++ b/include/linux/printk.h @@ -9,6 +9,8 @@ #include #include +struct console; + extern const char linux_banner[]; extern const char linux_proc_banner[]; @@ -198,6 +200,8 @@ extern asmlinkage void dump_stack_lvl(co extern asmlinkage void dump_stack(void) __cold; void printk_trigger_flush(void); void console_try_replay_all(void); +extern bool nbcon_device_try_acquire(struct console *con); +extern void nbcon_device_release(struct console *con); #else static inline __printf(1, 0) int vprintk(const char *s, va_list args) @@ -280,6 +284,16 @@ static inline void printk_trigger_flush( static inline void console_try_replay_all(void) { } + +static inline bool nbcon_device_try_acquire(struct console *con) +{ + return false; +} + +static inline void nbcon_device_release(struct console *con) +{ +} + #endif bool this_cpu_in_panic(void); --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -5,7 +5,9 @@ #include #include #include +#include #include +#include #include "internal.h" /* * Printk console printing implementation for consoles which does not depend @@ -546,6 +548,7 @@ static struct printk_buffers panic_nbcon * nbcon_context_try_acquire - Try to acquire nbcon console * @ctxt: The context of the caller * + * Context: Under @ctxt->con->device_lock() or local_irq_save(). * Return: True if the console was acquired. False otherwise. * * If the caller allowed an unsafe hostile takeover, on success the @@ -553,7 +556,6 @@ static struct printk_buffers panic_nbcon * in an unsafe state. Otherwise, on success the caller may assume * the console is not in an unsafe state. */ -__maybe_unused static bool nbcon_context_try_acquire(struct nbcon_context *ctxt) { unsigned int cpu = smp_processor_id(); @@ -1011,3 +1013,57 @@ void nbcon_free(struct console *con) con->pbufs = NULL; } + +/** + * nbcon_device_try_acquire - Try to acquire nbcon console and enter unsafe + * section + * @con: The nbcon console to acquire + * + * Context: Under the locking mechanism implemented in + * @con->device_lock() including disabling migration. + * Return: True if the console was acquired. False otherwise. + * + * Console drivers will usually use their own internal synchronization + * mechasism to synchronize between console printing and non-printing + * activities (such as setting baud rates). However, nbcon console drivers + * supporting atomic consoles may also want to mark unsafe sections when + * performing non-printing activities in order to synchronize against their + * atomic_write() callback. + * + * This function acquires the nbcon console using priority NBCON_PRIO_NORMAL + * and marks it unsafe for handover/takeover. + */ +bool nbcon_device_try_acquire(struct console *con) +{ + struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_device_ctxt); + + cant_migrate(); + + memset(ctxt, 0, sizeof(*ctxt)); + ctxt->console = con; + ctxt->prio = NBCON_PRIO_NORMAL; + + if (!nbcon_context_try_acquire(ctxt)) + return false; + + if (!nbcon_context_enter_unsafe(ctxt)) + return false; + + return true; +} +EXPORT_SYMBOL_GPL(nbcon_device_try_acquire); + +/** + * nbcon_device_release - Exit unsafe section and release the nbcon console + * @con: The nbcon console acquired in nbcon_device_try_acquire() + */ +void nbcon_device_release(struct console *con) +{ + struct nbcon_context *ctxt = &ACCESS_PRIVATE(con, nbcon_device_ctxt); + + if (!nbcon_context_exit_unsafe(ctxt)) + return; + + nbcon_context_release(ctxt); +} +EXPORT_SYMBOL_GPL(nbcon_device_release);