Commit 06653d57 authored by Thomas Gleixner's avatar Thomas Gleixner Committed by Petr Mladek
Browse files

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: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarJohn Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarThomas Gleixner (Intel) <tglx@linutronix.de>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20230916192007.608398-8-john.ogness@linutronix.de
parent ad56ebd1

include/linux/console.h

+21 −0
Original line number Diff line number Diff line
@@ -242,6 +242,7 @@ 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

 */
@@ -252,11 +253,28 @@ 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
@@ -277,6 +295,7 @@ 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
@@ -301,6 +320,8 @@ 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;

kernel/printk/internal.h

+6 −0
Original line number Diff line number Diff line
@@ -130,3 +130,9 @@ 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

kernel/printk/nbcon.c

+104 −2
Original line number Diff line number Diff line
@@ -221,7 +221,6 @@ void nbcon_seq_force(struct console *con, u64 seq) 
 * 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);
@@ -755,7 +754,6 @@ 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;
@@ -784,6 +782,110 @@ static bool __nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsaf 
	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;



	/*

	 * 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

kernel/printk/printk.c

+3 −6
Original line number Diff line number Diff line
@@ -698,9 +698,6 @@ 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;
@@ -2733,7 +2730,7 @@ 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);
@@ -2787,7 +2784,7 @@ 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 printk_get_next_message(struct printk_message *pmsg, u64 seq,

			     bool is_extended, bool may_suppress)

{

	static int panic_console_dropped;