From 218e8b89269e9dade9d8e9da6c86db88315c5ce2 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner <tglx@linutronix.de> Date: Sat, 16 Sep 2023 21:26:06 +0206 Subject: [PATCH 107/204] printk: nbcon: Add emit function and callback function for atomic printing Implement an emit function for nbcon consoles to output printk messages. It utilizes the lockless printk_get_next_message() and console_prepend_dropped() functions to retrieve/build the output message. The emit function includes the required safety points to check for handover/takeover and calls a new write_atomic callback of the console driver to output the message. It also includes proper handling for updating the nbcon console sequence number. A new nbcon_write_context struct is introduced. This is provided to the write_atomic callback and includes only the information necessary for performing atomic writes. Co-developed-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: John Ogness <john.ogness@linutronix.de> Signed-off-by: Thomas Gleixner (Intel) <tglx@linutronix.de> Reviewed-by: Petr Mladek <pmladek@suse.com> Signed-off-by: Petr Mladek <pmladek@suse.com> Link: https://lore.kernel.org/r/20230916192007.608398-8-john.ogness@linutronix.de Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> --- include/linux/console.h | 21 +++++++++ kernel/printk/internal.h | 6 ++ kernel/printk/nbcon.c | 106 ++++++++++++++++++++++++++++++++++++++++++++++- kernel/printk/printk.c | 9 +-- 4 files changed, 134 insertions(+), 8 deletions(-) Index: linux-6.6.58-rt45/include/linux/console.h =================================================================== @ linux-6.6.58-rt45/include/linux/console.h:245 @ struct printk_buffers; * be used only with NBCON_PRIO_PANIC @prio. It * might cause a system freeze when the console * is used later. + * @backlog: Ringbuffer has pending records * @pbufs: Pointer to the text buffer for this context * @seq: The sequence number to print for this context */ @ linux-6.6.58-rt45/include/linux/console.h:256 @ struct nbcon_context { enum nbcon_prio prio; unsigned int allow_unsafe_takeover : 1; + /* members set by emit */ + unsigned int backlog : 1; + /* members set by acquire */ struct printk_buffers *pbufs; u64 seq; }; /** + * struct nbcon_write_context - Context handed to the nbcon write callbacks + * @ctxt: The core console context + * @outbuf: Pointer to the text buffer for output + * @len: Length to write + * @unsafe_takeover: If a hostile takeover in an unsafe state has occurred + */ +struct nbcon_write_context { + struct nbcon_context __private ctxt; + char *outbuf; + unsigned int len; + bool unsafe_takeover; +}; + +/** * struct console - The console descriptor structure * @name: The name of the console driver * @write: Write callback to output messages (Optional) @ linux-6.6.58-rt45/include/linux/console.h:298 @ struct nbcon_context { * @data: Driver private data * @node: hlist node for the console list * + * @write_atomic: Write callback for atomic context * @nbcon_state: State for nbcon consoles * @nbcon_seq: Sequence number of the next record for nbcon to print * @pbufs: Pointer to nbcon private buffer @ linux-6.6.58-rt45/include/linux/console.h:323 @ struct console { struct hlist_node node; /* nbcon console specific members */ + bool (*write_atomic)(struct console *con, + struct nbcon_write_context *wctxt); atomic_t __private nbcon_state; atomic_long_t __private nbcon_seq; struct printk_buffers *pbufs; Index: linux-6.6.58-rt45/kernel/printk/internal.h =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/internal.h +++ linux-6.6.58-rt45/kernel/printk/internal.h @ linux-6.6.58-rt45/include/linux/console.h:133 @ struct printk_message { }; bool other_cpu_in_panic(void); +bool printk_get_next_message(struct printk_message *pmsg, u64 seq, + bool is_extended, bool may_supress); + +#ifdef CONFIG_PRINTK +void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped); +#endif Index: linux-6.6.58-rt45/kernel/printk/nbcon.c =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/nbcon.c +++ linux-6.6.58-rt45/kernel/printk/nbcon.c @ linux-6.6.58-rt45/include/linux/console.h:224 @ void nbcon_seq_force(struct console *con * nbcon_seq_force() was used or the current context no longer owns the * console. In the later case, it will stop printing anyway. */ -__maybe_unused static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq) { unsigned long nbcon_seq = __seq_to_nbcon_seq(ctxt->seq); @ linux-6.6.58-rt45/include/linux/console.h:757 @ static bool nbcon_context_can_proceed(st * * Internal helper to avoid duplicated code. */ -__maybe_unused static bool __nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe) { struct console *con = ctxt->console; @ linux-6.6.58-rt45/include/linux/console.h:786 @ out: } /** + * nbcon_emit_next_record - Emit a record in the acquired context + * @wctxt: The write context that will be handed to the write function + * + * Return: True if this context still owns the console. False if + * ownership was handed over or taken. + * + * When this function returns false then the calling context no longer owns + * the console and is no longer allowed to go forward. In this case it must + * back out immediately and carefully. The buffer content is also no longer + * trusted since it no longer belongs to the calling context. If the caller + * wants to do more it must reacquire the console first. + * + * When true is returned, @wctxt->ctxt.backlog indicates whether there are + * still records pending in the ringbuffer, + */ +__maybe_unused +static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt) +{ + struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt); + struct console *con = ctxt->console; + bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED; + struct printk_message pmsg = { + .pbufs = ctxt->pbufs, + }; + unsigned long con_dropped; + struct nbcon_state cur; + unsigned long dropped; + bool done; + + /* + * The printk buffers are filled within an unsafe section. This + * prevents NBCON_PRIO_NORMAL and NBCON_PRIO_EMERGENCY from + * clobbering each other. + */ + + if (!nbcon_context_enter_unsafe(ctxt)) + return false; + + ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended, true); + if (!ctxt->backlog) + return nbcon_context_exit_unsafe(ctxt); + + /* + * @con->dropped is not protected in case of an unsafe hostile + * takeover. In that situation the update can be racy so + * annotate it accordingly. + */ + con_dropped = data_race(READ_ONCE(con->dropped)); + + dropped = con_dropped + pmsg.dropped; + if (dropped && !is_extended) + console_prepend_dropped(&pmsg, dropped); + + if (!nbcon_context_exit_unsafe(ctxt)) + return false; + + /* For skipped records just update seq/dropped in @con. */ + if (pmsg.outbuf_len == 0) + goto update_con; + + /* Initialize the write context for driver callbacks. */ + wctxt->outbuf = &pmsg.pbufs->outbuf[0]; + wctxt->len = pmsg.outbuf_len; + nbcon_state_read(con, &cur); + wctxt->unsafe_takeover = cur.unsafe_takeover; + + if (con->write_atomic) { + done = con->write_atomic(con, wctxt); + } else { + nbcon_context_release(ctxt); + WARN_ON_ONCE(1); + done = false; + } + + /* If not done, the emit was aborted. */ + if (!done) + return false; + + /* + * Since any dropped message was successfully output, reset the + * dropped count for the console. + */ + dropped = 0; +update_con: + /* + * The dropped count and the sequence number are updated within an + * unsafe section. This limits update races to the panic context and + * allows the panic context to win. + */ + + if (!nbcon_context_enter_unsafe(ctxt)) + return false; + + if (dropped != con_dropped) { + /* Counterpart to the READ_ONCE() above. */ + WRITE_ONCE(con->dropped, dropped); + } + + nbcon_seq_try_update(ctxt, pmsg.seq + 1); + + return nbcon_context_exit_unsafe(ctxt); +} + +/** * nbcon_alloc - Allocate buffers needed by the nbcon console * @con: Console to allocate buffers for * Index: linux-6.6.58-rt45/kernel/printk/printk.c =================================================================== --- linux-6.6.58-rt45.orig/kernel/printk/printk.c +++ linux-6.6.58-rt45/kernel/printk/printk.c @ linux-6.6.58-rt45/include/linux/console.h:701 @ out: return len; } -static bool printk_get_next_message(struct printk_message *pmsg, u64 seq, - bool is_extended, bool may_supress); - /* /dev/kmsg - userspace message inject/listen interface */ struct devkmsg_user { atomic64_t seq; @ linux-6.6.58-rt45/include/linux/console.h:2767 @ static void __console_unlock(void) * If @pmsg->pbufs->outbuf is modified, @pmsg->outbuf_len is updated. */ #ifdef CONFIG_PRINTK -static void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped) +void console_prepend_dropped(struct printk_message *pmsg, unsigned long dropped) { struct printk_buffers *pbufs = pmsg->pbufs; const size_t scratchbuf_sz = sizeof(pbufs->scratchbuf); @ linux-6.6.58-rt45/include/linux/console.h:2821 @ static void console_prepend_dropped(stru * of @pmsg are valid. (See the documentation of struct printk_message * for information about the @pmsg fields.) */ -static bool printk_get_next_message(struct printk_message *pmsg, u64 seq, - bool is_extended, bool may_suppress) +bool printk_get_next_message(struct printk_message *pmsg, u64 seq, + bool is_extended, bool may_suppress) { static int panic_console_dropped;