Messages in this thread Patch in this message | | | From | John Ogness <> | Subject | [PATCH printk v3 6/7] printk: nbcon: Add emit function and callback function for atomic printing | Date | Sun, 3 Sep 2023 17:11:38 +0206 |
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From: Thomas Gleixner <tglx@linutronix.de>
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> --- include/linux/console.h | 27 +++++++++ kernel/printk/internal.h | 6 ++ kernel/printk/nbcon.c | 124 ++++++++++++++++++++++++++++++++++++++- kernel/printk/printk.c | 9 +-- 4 files changed, 158 insertions(+), 8 deletions(-)
diff --git a/include/linux/console.h b/include/linux/console.h index 8e8dc8d1016b..8d0a309b5286 100644 --- a/include/linux/console.h +++ b/include/linux/console.h @@ -248,6 +248,7 @@ 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 */ @@ -258,11 +259,28 @@ 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 @@ -283,6 +301,7 @@ 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 @@ -307,6 +326,8 @@ 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; @@ -436,6 +457,12 @@ static inline bool console_is_registered(const struct console *con) lockdep_assert_console_list_lock_held(); \ hlist_for_each_entry(con, &console_list, node) +#ifdef CONFIG_PRINTK +extern bool nbcon_can_proceed(struct nbcon_write_context *wctxt); +#else +static inline bool nbcon_can_proceed(struct nbcon_write_context *wctxt) { return false; } +#endif + extern int console_set_on_cmdline; extern struct console *early_console; diff --git a/kernel/printk/internal.h b/kernel/printk/internal.h index 956f65308392..4d532623b3cf 100644 --- a/kernel/printk/internal.h +++ b/kernel/printk/internal.h @@ -128,3 +128,9 @@ 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 diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c index cd8d1e8539af..1a3df603111d 100644 --- a/kernel/printk/nbcon.c +++ b/kernel/printk/nbcon.c @@ -176,7 +176,6 @@ void nbcon_seq_force(struct console *con, u64 seq) * 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); @@ -684,6 +683,32 @@ static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_s return false; } +/** + * nbcon_can_proceed - Check whether ownership can proceed + * @wctxt: The write context that was handed to the write function + * + * Return: True if this context still owns the console. False if + * ownership was handed over or taken. + * + * Must be invoked at appropriate safe places in the driver. + * + * 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. + */ +bool nbcon_can_proceed(struct nbcon_write_context *wctxt) +{ + struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt); + struct console *con = ctxt->console; + struct nbcon_state cur; + + nbcon_state_read(con, &cur); + + return nbcon_context_can_proceed(ctxt, &cur); +} +EXPORT_SYMBOL_GPL(nbcon_can_proceed); + /** * nbcon_context_update_unsafe - Update the unsafe bit in @con->nbcon_state * @ctxt: The nbcon context from nbcon_context_try_acquire() @@ -703,7 +728,6 @@ static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_s * * 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; @@ -732,6 +756,102 @@ static bool nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe) return nbcon_context_can_proceed(ctxt, &cur); } +/** + * 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; + + ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended, true); + if (!ctxt->backlog) + return true; + + /* + * @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); + + /* Safety point. Do not touch state in case of takeover. */ + nbcon_state_read(con, &cur); + if (!nbcon_context_can_proceed(ctxt, &cur)) + return false; + + /* For skipped records just update seq/dropped in @con. */ + if (pmsg.outbuf_len == 0) + goto update_con; + + /* Set the write context before calling write callback. */ + wctxt->outbuf = &pmsg.pbufs->outbuf[0]; + wctxt->len = pmsg.outbuf_len; + 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_update_unsafe(ctxt, true)) + 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_update_unsafe(ctxt, false); +} + /** * nbcon_alloc - Allocate buffers needed by the nbcon console * @con: Console to initialize diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 4e22ff8d065e..f6e1fca2a2cb 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -696,9 +696,6 @@ static ssize_t msg_print_ext_body(char *buf, size_t size, 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; @@ -2731,7 +2728,7 @@ 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); @@ -2785,8 +2782,8 @@ static void console_prepend_dropped(struct printk_message *pmsg, unsigned long d * 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; -- 2.39.2
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