Merge branch 'rework/nbcon-base' into for-linus

1 files changed, 1029 insertions, 0 deletions

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
new file mode 100644
index 000000000000..b96077152f49
--- /dev/null
+++ b/kernel/printk/nbcon.c
@@ -0,0 +1,1029 @@
+// SPDX-License-Identifier: GPL-2.0-only
+// Copyright (C) 2022 Linutronix GmbH, John Ogness
+// Copyright (C) 2022 Intel, Thomas Gleixner
+
+#include <linux/kernel.h>
+#include <linux/console.h>
+#include <linux/delay.h>
+#include <linux/slab.h>
+#include "internal.h"
+/*
+ * Printk console printing implementation for consoles which does not depend
+ * on the legacy style console_lock mechanism.
+ *
+ * The state of the console is maintained in the "nbcon_state" atomic
+ * variable.
+ *
+ * The console is locked when:
+ *
+ *   - The 'prio' field contains the priority of the context that owns the
+ *     console. Only higher priority contexts are allowed to take over the
+ *     lock. A value of 0 (NBCON_PRIO_NONE) means the console is not locked.
+ *
+ *   - The 'cpu' field denotes on which CPU the console is locked. It is used
+ *     to prevent busy waiting on the same CPU. Also it informs the lock owner
+ *     that it has lost the lock in a more complex scenario when the lock was
+ *     taken over by a higher priority context, released, and taken on another
+ *     CPU with the same priority as the interrupted owner.
+ *
+ * The acquire mechanism uses a few more fields:
+ *
+ *   - The 'req_prio' field is used by the handover approach to make the
+ *     current owner aware that there is a context with a higher priority
+ *     waiting for the friendly handover.
+ *
+ *   - The 'unsafe' field allows to take over the console in a safe way in the
+ *     middle of emitting a message. The field is set only when accessing some
+ *     shared resources or when the console device is manipulated. It can be
+ *     cleared, for example, after emitting one character when the console
+ *     device is in a consistent state.
+ *
+ *   - The 'unsafe_takeover' field is set when a hostile takeover took the
+ *     console in an unsafe state. The console will stay in the unsafe state
+ *     until re-initialized.
+ *
+ * The acquire mechanism uses three approaches:
+ *
+ *   1) Direct acquire when the console is not owned or is owned by a lower
+ *      priority context and is in a safe state.
+ *
+ *   2) Friendly handover mechanism uses a request/grant handshake. It is used
+ *      when the current owner has lower priority and the console is in an
+ *      unsafe state.
+ *
+ *      The requesting context:
+ *
+ *        a) Sets its priority into the 'req_prio' field.
+ *
+ *        b) Waits (with a timeout) for the owning context to unlock the
+ *           console.
+ *
+ *        c) Takes the lock and clears the 'req_prio' field.
+ *
+ *      The owning context:
+ *
+ *        a) Observes the 'req_prio' field set on exit from the unsafe
+ *           console state.
+ *
+ *        b) Gives up console ownership by clearing the 'prio' field.
+ *
+ *   3) Unsafe hostile takeover allows to take over the lock even when the
+ *      console is an unsafe state. It is used only in panic() by the final
+ *      attempt to flush consoles in a try and hope mode.
+ *
+ *      Note that separate record buffers are used in panic(). As a result,
+ *      the messages can be read and formatted without any risk even after
+ *      using the hostile takeover in unsafe state.
+ *
+ * The release function simply clears the 'prio' field.
+ *
+ * All operations on @console::nbcon_state are atomic cmpxchg based to
+ * handle concurrency.
+ *
+ * The acquire/release functions implement only minimal policies:
+ *
+ *   - Preference for higher priority contexts.
+ *   - Protection of the panic CPU.
+ *
+ * All other policy decisions must be made at the call sites:
+ *
+ *   - What is marked as an unsafe section.
+ *   - Whether to spin-wait if there is already an owner and the console is
+ *     in an unsafe state.
+ *   - Whether to attempt an unsafe hostile takeover.
+ *
+ * The design allows to implement the well known:
+ *
+ *     acquire()
+ *     output_one_printk_record()
+ *     release()
+ *
+ * The output of one printk record might be interrupted with a higher priority
+ * context. The new owner is supposed to reprint the entire interrupted record
+ * from scratch.
+ */
+
+/**
+ * nbcon_state_set - Helper function to set the console state
+ * @con:	Console to update
+ * @new:	The new state to write
+ *
+ * Only to be used when the console is not yet or no longer visible in the
+ * system. Otherwise use nbcon_state_try_cmpxchg().
+ */
+static inline void nbcon_state_set(struct console *con, struct nbcon_state *new)
+{
+	atomic_set(&ACCESS_PRIVATE(con, nbcon_state), new->atom);
+}
+
+/**
+ * nbcon_state_read - Helper function to read the console state
+ * @con:	Console to read
+ * @state:	The state to store the result
+ */
+static inline void nbcon_state_read(struct console *con, struct nbcon_state *state)
+{
+	state->atom = atomic_read(&ACCESS_PRIVATE(con, nbcon_state));
+}
+
+/**
+ * nbcon_state_try_cmpxchg() - Helper function for atomic_try_cmpxchg() on console state
+ * @con:	Console to update
+ * @cur:	Old/expected state
+ * @new:	New state
+ *
+ * Return: True on success. False on fail and @cur is updated.
+ */
+static inline bool nbcon_state_try_cmpxchg(struct console *con, struct nbcon_state *cur,
+					   struct nbcon_state *new)
+{
+	return atomic_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_state), &cur->atom, new->atom);
+}
+
+#ifdef CONFIG_64BIT
+
+#define __seq_to_nbcon_seq(seq) (seq)
+#define __nbcon_seq_to_seq(seq) (seq)
+
+#else /* CONFIG_64BIT */
+
+#define __seq_to_nbcon_seq(seq) ((u32)seq)
+
+static inline u64 __nbcon_seq_to_seq(u32 nbcon_seq)
+{
+	u64 seq;
+	u64 rb_next_seq;
+
+	/*
+	 * The provided sequence is only the lower 32 bits of the ringbuffer
+	 * sequence. It needs to be expanded to 64bit. Get the next sequence
+	 * number from the ringbuffer and fold it.
+	 *
+	 * Having a 32bit representation in the console is sufficient.
+	 * If a console ever gets more than 2^31 records behind
+	 * the ringbuffer then this is the least of the problems.
+	 *
+	 * Also the access to the ring buffer is always safe.
+	 */
+	rb_next_seq = prb_next_seq(prb);
+	seq = rb_next_seq - ((u32)rb_next_seq - nbcon_seq);
+
+	return seq;
+}
+
+#endif /* CONFIG_64BIT */
+
+/**
+ * nbcon_seq_read - Read the current console sequence
+ * @con:	Console to read the sequence of
+ *
+ * Return:	Sequence number of the next record to print on @con.
+ */
+u64 nbcon_seq_read(struct console *con)
+{
+	unsigned long nbcon_seq = atomic_long_read(&ACCESS_PRIVATE(con, nbcon_seq));
+
+	return __nbcon_seq_to_seq(nbcon_seq);
+}
+
+/**
+ * nbcon_seq_force - Force console sequence to a specific value
+ * @con:	Console to work on
+ * @seq:	Sequence number value to set
+ *
+ * Only to be used during init (before registration) or in extreme situations
+ * (such as panic with CONSOLE_REPLAY_ALL).
+ */
+void nbcon_seq_force(struct console *con, u64 seq)
+{
+	/*
+	 * If the specified record no longer exists, the oldest available record
+	 * is chosen. This is especially important on 32bit systems because only
+	 * the lower 32 bits of the sequence number are stored. The upper 32 bits
+	 * are derived from the sequence numbers available in the ringbuffer.
+	 */
+	u64 valid_seq = max_t(u64, seq, prb_first_valid_seq(prb));
+
+	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_seq), __seq_to_nbcon_seq(valid_seq));
+
+	/* Clear con->seq since nbcon consoles use con->nbcon_seq instead. */
+	con->seq = 0;
+}
+
+/**
+ * nbcon_seq_try_update - Try to update the console sequence number
+ * @ctxt:	Pointer to an acquire context that contains
+ *		all information about the acquire mode
+ * @new_seq:	The new sequence number to set
+ *
+ * @ctxt->seq is updated to the new value of @con::nbcon_seq (expanded to
+ * the 64bit value). This could be a different value than @new_seq if
+ * nbcon_seq_force() was used or the current context no longer owns the
+ * console. In the later case, it will stop printing anyway.
+ */
+static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq)
+{
+	unsigned long nbcon_seq = __seq_to_nbcon_seq(ctxt->seq);
+	struct console *con = ctxt->console;
+
+	if (atomic_long_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_seq), &nbcon_seq,
+				    __seq_to_nbcon_seq(new_seq))) {
+		ctxt->seq = new_seq;
+	} else {
+		ctxt->seq = nbcon_seq_read(con);
+	}
+}
+
+/**
+ * nbcon_context_try_acquire_direct - Try to acquire directly
+ * @ctxt:	The context of the caller
+ * @cur:	The current console state
+ *
+ * Acquire the console when it is released. Also acquire the console when
+ * the current owner has a lower priority and the console is in a safe state.
+ *
+ * Return:	0 on success. Otherwise, an error code on failure. Also @cur
+ *		is updated to the latest state when failed to modify it.
+ *
+ * Errors:
+ *
+ *	-EPERM:		A panic is in progress and this is not the panic CPU.
+ *			Or the current owner or waiter has the same or higher
+ *			priority. No acquire method can be successful in
+ *			this case.
+ *
+ *	-EBUSY:		The current owner has a lower priority but the console
+ *			in an unsafe state. The caller should try using
+ *			the handover acquire method.
+ */
+static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
+					    struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+	struct console *con = ctxt->console;
+	struct nbcon_state new;
+
+	do {
+		if (other_cpu_in_panic())
+			return -EPERM;
+
+		if (ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio)
+			return -EPERM;
+
+		if (cur->unsafe)
+			return -EBUSY;
+
+		/*
+		 * The console should never be safe for a direct acquire
+		 * if an unsafe hostile takeover has ever happened.
+		 */
+		WARN_ON_ONCE(cur->unsafe_takeover);
+
+		new.atom = cur->atom;
+		new.prio	= ctxt->prio;
+		new.req_prio	= NBCON_PRIO_NONE;
+		new.unsafe	= cur->unsafe_takeover;
+		new.cpu		= cpu;
+
+	} while (!nbcon_state_try_cmpxchg(con, cur, &new));
+
+	return 0;
+}
+
+static bool nbcon_waiter_matches(struct nbcon_state *cur, int expected_prio)
+{
+	/*
+	 * The request context is well defined by the @req_prio because:
+	 *
+	 * - Only a context with a higher priority can take over the request.
+	 * - There are only three priorities.
+	 * - Only one CPU is allowed to request PANIC priority.
+	 * - Lower priorities are ignored during panic() until reboot.
+	 *
+	 * As a result, the following scenario is *not* possible:
+	 *
+	 * 1. Another context with a higher priority directly takes ownership.
+	 * 2. The higher priority context releases the ownership.
+	 * 3. A lower priority context takes the ownership.
+	 * 4. Another context with the same priority as this context
+	 *    creates a request and starts waiting.
+	 */
+
+	return (cur->req_prio == expected_prio);
+}
+
+/**
+ * nbcon_context_try_acquire_requested - Try to acquire after having
+ *					 requested a handover
+ * @ctxt:	The context of the caller
+ * @cur:	The current console state
+ *
+ * This is a helper function for nbcon_context_try_acquire_handover().
+ * It is called when the console is in an unsafe state. The current
+ * owner will release the console on exit from the unsafe region.
+ *
+ * Return:	0 on success and @cur is updated to the new console state.
+ *		Otherwise an error code on failure.
+ *
+ * Errors:
+ *
+ *	-EPERM:		A panic is in progress and this is not the panic CPU
+ *			or this context is no longer the waiter.
+ *
+ *	-EBUSY:		The console is still locked. The caller should
+ *			continue waiting.
+ *
+ * Note: The caller must still remove the request when an error has occurred
+ *       except when this context is no longer the waiter.
+ */
+static int nbcon_context_try_acquire_requested(struct nbcon_context *ctxt,
+					       struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+	struct console *con = ctxt->console;
+	struct nbcon_state new;
+
+	/* Note that the caller must still remove the request! */
+	if (other_cpu_in_panic())
+		return -EPERM;
+
+	/*
+	 * Note that the waiter will also change if there was an unsafe
+	 * hostile takeover.
+	 */
+	if (!nbcon_waiter_matches(cur, ctxt->prio))
+		return -EPERM;
+
+	/* If still locked, caller should continue waiting. */
+	if (cur->prio != NBCON_PRIO_NONE)
+		return -EBUSY;
+
+	/*
+	 * The previous owner should have never released ownership
+	 * in an unsafe region.
+	 */
+	WARN_ON_ONCE(cur->unsafe);
+
+	new.atom = cur->atom;
+	new.prio	= ctxt->prio;
+	new.req_prio	= NBCON_PRIO_NONE;
+	new.unsafe	= cur->unsafe_takeover;
+	new.cpu		= cpu;
+
+	if (!nbcon_state_try_cmpxchg(con, cur, &new)) {
+		/*
+		 * The acquire could fail only when it has been taken
+		 * over by a higher priority context.
+		 */
+		WARN_ON_ONCE(nbcon_waiter_matches(cur, ctxt->prio));
+		return -EPERM;
+	}
+
+	/* Handover success. This context now owns the console. */
+	return 0;
+}
+
+/**
+ * nbcon_context_try_acquire_handover - Try to acquire via handover
+ * @ctxt:	The context of the caller
+ * @cur:	The current console state
+ *
+ * The function must be called only when the context has higher priority
+ * than the current owner and the console is in an unsafe state.
+ * It is the case when nbcon_context_try_acquire_direct() returns -EBUSY.
+ *
+ * The function sets "req_prio" field to make the current owner aware of
+ * the request. Then it waits until the current owner releases the console,
+ * or an even higher context takes over the request, or timeout expires.
+ *
+ * The current owner checks the "req_prio" field on exit from the unsafe
+ * region and releases the console. It does not touch the "req_prio" field
+ * so that the console stays reserved for the waiter.
+ *
+ * Return:	0 on success. Otherwise, an error code on failure. Also @cur
+ *		is updated to the latest state when failed to modify it.
+ *
+ * Errors:
+ *
+ *	-EPERM:		A panic is in progress and this is not the panic CPU.
+ *			Or a higher priority context has taken over the
+ *			console or the handover request.
+ *
+ *	-EBUSY:		The current owner is on the same CPU so that the hand
+ *			shake could not work. Or the current owner is not
+ *			willing to wait (zero timeout). Or the console does
+ *			not enter the safe state before timeout passed. The
+ *			caller might still use the unsafe hostile takeover
+ *			when allowed.
+ *
+ *	-EAGAIN:	@cur has changed when creating the handover request.
+ *			The caller should retry with direct acquire.
+ */
+static int nbcon_context_try_acquire_handover(struct nbcon_context *ctxt,
+					      struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+	struct console *con = ctxt->console;
+	struct nbcon_state new;
+	int timeout;
+	int request_err = -EBUSY;
+
+	/*
+	 * Check that the handover is called when the direct acquire failed
+	 * with -EBUSY.
+	 */
+	WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio);
+	WARN_ON_ONCE(!cur->unsafe);
+
+	/* Handover is not possible on the same CPU. */
+	if (cur->cpu == cpu)
+		return -EBUSY;
+
+	/*
+	 * Console stays unsafe after an unsafe takeover until re-initialized.
+	 * Waiting is not going to help in this case.
+	 */
+	if (cur->unsafe_takeover)
+		return -EBUSY;
+
+	/* Is the caller willing to wait? */
+	if (ctxt->spinwait_max_us == 0)
+		return -EBUSY;
+
+	/*
+	 * Setup a request for the handover. The caller should try to acquire
+	 * the console directly when the current state has been modified.
+	 */
+	new.atom = cur->atom;
+	new.req_prio = ctxt->prio;
+	if (!nbcon_state_try_cmpxchg(con, cur, &new))
+		return -EAGAIN;
+
+	cur->atom = new.atom;
+
+	/* Wait until there is no owner and then acquire the console. */
+	for (timeout = ctxt->spinwait_max_us; timeout >= 0; timeout--) {
+		/* On successful acquire, this request is cleared. */
+		request_err = nbcon_context_try_acquire_requested(ctxt, cur);
+		if (!request_err)
+			return 0;
+
+		/*
+		 * If the acquire should be aborted, it must be ensured
+		 * that the request is removed before returning to caller.
+		 */
+		if (request_err == -EPERM)
+			break;
+
+		udelay(1);
+
+		/* Re-read the state because some time has passed. */
+		nbcon_state_read(con, cur);
+	}
+
+	/* Timed out or aborted. Carefully remove handover request. */
+	do {
+		/*
+		 * No need to remove request if there is a new waiter. This
+		 * can only happen if a higher priority context has taken over
+		 * the console or the handover request.
+		 */
+		if (!nbcon_waiter_matches(cur, ctxt->prio))
+			return -EPERM;
+
+		/* Unset request for handover. */
+		new.atom = cur->atom;
+		new.req_prio = NBCON_PRIO_NONE;
+		if (nbcon_state_try_cmpxchg(con, cur, &new)) {
+			/*
+			 * Request successfully unset. Report failure of
+			 * acquiring via handover.
+			 */
+			cur->atom = new.atom;
+			return request_err;
+		}
+
+		/*
+		 * Unable to remove request. Try to acquire in case
+		 * the owner has released the lock.
+		 */
+	} while (nbcon_context_try_acquire_requested(ctxt, cur));
+
+	/* Lucky timing. The acquire succeeded while removing the request. */
+	return 0;
+}
+
+/**
+ * nbcon_context_try_acquire_hostile - Acquire via unsafe hostile takeover
+ * @ctxt:	The context of the caller
+ * @cur:	The current console state
+ *
+ * Acquire the console even in the unsafe state.
+ *
+ * It can be permitted by setting the 'allow_unsafe_takeover' field only
+ * by the final attempt to flush messages in panic().
+ *
+ * Return:	0 on success. -EPERM when not allowed by the context.
+ */
+static int nbcon_context_try_acquire_hostile(struct nbcon_context *ctxt,
+					     struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+	struct console *con = ctxt->console;
+	struct nbcon_state new;
+
+	if (!ctxt->allow_unsafe_takeover)
+		return -EPERM;
+
+	/* Ensure caller is allowed to perform unsafe hostile takeovers. */
+	if (WARN_ON_ONCE(ctxt->prio != NBCON_PRIO_PANIC))
+		return -EPERM;
+
+	/*
+	 * Check that try_acquire_direct() and try_acquire_handover() returned
+	 * -EBUSY in the right situation.
+	 */
+	WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio);
+	WARN_ON_ONCE(cur->unsafe != true);
+
+	do {
+		new.atom = cur->atom;
+		new.cpu			= cpu;
+		new.prio		= ctxt->prio;
+		new.unsafe		|= cur->unsafe_takeover;
+		new.unsafe_takeover	|= cur->unsafe;
+
+	} while (!nbcon_state_try_cmpxchg(con, cur, &new));
+
+	return 0;
+}
+
+static struct printk_buffers panic_nbcon_pbufs;
+
+/**
+ * nbcon_context_try_acquire - Try to acquire nbcon console
+ * @ctxt:	The context of the caller
+ *
+ * Return:	True if the console was acquired. False otherwise.
+ *
+ * If the caller allowed an unsafe hostile takeover, on success the
+ * caller should check the current console state to see if it is
+ * 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();
+	struct console *con = ctxt->console;
+	struct nbcon_state cur;
+	int err;
+
+	nbcon_state_read(con, &cur);
+try_again:
+	err = nbcon_context_try_acquire_direct(ctxt, &cur);
+	if (err != -EBUSY)
+		goto out;
+
+	err = nbcon_context_try_acquire_handover(ctxt, &cur);
+	if (err == -EAGAIN)
+		goto try_again;
+	if (err != -EBUSY)
+		goto out;
+
+	err = nbcon_context_try_acquire_hostile(ctxt, &cur);
+out:
+	if (err)
+		return false;
+
+	/* Acquire succeeded. */
+
+	/* Assign the appropriate buffer for this context. */
+	if (atomic_read(&panic_cpu) == cpu)
+		ctxt->pbufs = &panic_nbcon_pbufs;
+	else
+		ctxt->pbufs = con->pbufs;
+
+	/* Set the record sequence for this context to print. */
+	ctxt->seq = nbcon_seq_read(ctxt->console);
+
+	return true;
+}
+
+static bool nbcon_owner_matches(struct nbcon_state *cur, int expected_cpu,
+				int expected_prio)
+{
+	/*
+	 * Since consoles can only be acquired by higher priorities,
+	 * owning contexts are uniquely identified by @prio. However,
+	 * since contexts can unexpectedly lose ownership, it is
+	 * possible that later another owner appears with the same
+	 * priority. For this reason @cpu is also needed.
+	 */
+
+	if (cur->prio != expected_prio)
+		return false;
+
+	if (cur->cpu != expected_cpu)
+		return false;
+
+	return true;
+}
+
+/**
+ * nbcon_context_release - Release the console
+ * @ctxt:	The nbcon context from nbcon_context_try_acquire()
+ */
+static void nbcon_context_release(struct nbcon_context *ctxt)
+{
+	unsigned int cpu = smp_processor_id();
+	struct console *con = ctxt->console;
+	struct nbcon_state cur;
+	struct nbcon_state new;
+
+	nbcon_state_read(con, &cur);
+
+	do {
+		if (!nbcon_owner_matches(&cur, cpu, ctxt->prio))
+			break;
+
+		new.atom = cur.atom;
+		new.prio = NBCON_PRIO_NONE;
+
+		/*
+		 * If @unsafe_takeover is set, it is kept set so that
+		 * the state remains permanently unsafe.
+		 */
+		new.unsafe |= cur.unsafe_takeover;
+
+	} while (!nbcon_state_try_cmpxchg(con, &cur, &new));
+
+	ctxt->pbufs = NULL;
+}
+
+/**
+ * nbcon_context_can_proceed - Check whether ownership can proceed
+ * @ctxt:	The nbcon context from nbcon_context_try_acquire()
+ * @cur:	The current console state
+ *
+ * Return:	True if this context still owns the console. False if
+ *		ownership was handed over or taken.
+ *
+ * Must be invoked when entering the unsafe state to make sure that it still
+ * owns the lock. Also must be invoked when exiting the unsafe context
+ * to eventually free the lock for a higher priority context which asked
+ * for the friendly handover.
+ *
+ * It can be called inside an unsafe section when the console is just
+ * temporary in safe state instead of exiting and entering the unsafe
+ * state.
+ *
+ * Also it can be called in the safe context before doing an expensive
+ * safe operation. It does not make sense to do the operation when
+ * a higher priority context took the lock.
+ *
+ * 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.
+ */
+static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur)
+{
+	unsigned int cpu = smp_processor_id();
+
+	/* Make sure this context still owns the console. */
+	if (!nbcon_owner_matches(cur, cpu, ctxt->prio))
+		return false;
+
+	/* The console owner can proceed if there is no waiter. */
+	if (cur->req_prio == NBCON_PRIO_NONE)
+		return true;
+
+	/*
+	 * A console owner within an unsafe region is always allowed to
+	 * proceed, even if there are waiters. It can perform a handover
+	 * when exiting the unsafe region. Otherwise the waiter will
+	 * need to perform an unsafe hostile takeover.
+	 */
+	if (cur->unsafe)
+		return true;
+
+	/* Waiters always have higher priorities than owners. */
+	WARN_ON_ONCE(cur->req_prio <= cur->prio);
+
+	/*
+	 * Having a safe point for take over and eventually a few
+	 * duplicated characters or a full line is way better than a
+	 * hostile takeover. Post processing can take care of the garbage.
+	 * Release and hand over.
+	 */
+	nbcon_context_release(ctxt);
+
+	/*
+	 * It is not clear whether the waiter really took over ownership. The
+	 * outermost callsite must make the final decision whether console
+	 * ownership is needed for it to proceed. If yes, it must reacquire
+	 * ownership (possibly hostile) before carefully proceeding.
+	 *
+	 * The calling context no longer owns the console so go back all the
+	 * way instead of trying to implement reacquire heuristics in tons of
+	 * places.
+	 */
+	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.
+ *
+ * It is used in nbcon_enter_unsafe() to make sure that it still owns the
+ * lock. Also it is used in nbcon_exit_unsafe() to eventually free the lock
+ * for a higher priority context which asked for the friendly handover.
+ *
+ * It can be called inside an unsafe section when the console is just
+ * temporary in safe state instead of exiting and entering the unsafe state.
+ *
+ * Also it can be called in the safe context before doing an expensive safe
+ * operation. It does not make sense to do the operation when a higher
+ * priority context took the lock.
+ *
+ * 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);
+
+#define nbcon_context_enter_unsafe(c)	__nbcon_context_update_unsafe(c, true)
+#define nbcon_context_exit_unsafe(c)	__nbcon_context_update_unsafe(c, false)
+
+/**
+ * __nbcon_context_update_unsafe - Update the unsafe bit in @con->nbcon_state
+ * @ctxt:	The nbcon context from nbcon_context_try_acquire()
+ * @unsafe:	The new value for the unsafe bit
+ *
+ * Return:	True if the unsafe state was updated and this context still
+ *		owns the console. Otherwise false if ownership was handed
+ *		over or taken.
+ *
+ * This function allows console owners to modify the unsafe status of the
+ * console.
+ *
+ * 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.
+ *
+ * Internal helper to avoid duplicated code.
+ */
+static bool __nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
+{
+	struct console *con = ctxt->console;
+	struct nbcon_state cur;
+	struct nbcon_state new;
+
+	nbcon_state_read(con, &cur);
+
+	do {
+		/*
+		 * The unsafe bit must not be cleared if an
+		 * unsafe hostile takeover has occurred.
+		 */
+		if (!unsafe && cur.unsafe_takeover)
+			goto out;
+
+		if (!nbcon_context_can_proceed(ctxt, &cur))
+			return false;
+
+		new.atom = cur.atom;
+		new.unsafe = unsafe;
+	} while (!nbcon_state_try_cmpxchg(con, &cur, &new));
+
+	cur.atom = new.atom;
+out:
+	return nbcon_context_can_proceed(ctxt, &cur);
+}
+
+/**
+ * nbcon_enter_unsafe - Enter an unsafe region in the driver
+ * @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.
+ *
+ * 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_enter_unsafe(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	return nbcon_context_enter_unsafe(ctxt);
+}
+EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
+
+/**
+ * nbcon_exit_unsafe - Exit an unsafe region in the driver
+ * @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.
+ *
+ * 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_exit_unsafe(struct nbcon_write_context *wctxt)
+{
+	struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
+
+	return nbcon_context_exit_unsafe(ctxt);
+}
+EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
+
+/**
+ * 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
+ *
+ * Return:	True on success. False otherwise and the console cannot
+ *		be used.
+ *
+ * This is not part of nbcon_init() because buffer allocation must
+ * be performed earlier in the console registration process.
+ */
+bool nbcon_alloc(struct console *con)
+{
+	if (con->flags & CON_BOOT) {
+		/*
+		 * Boot console printing is synchronized with legacy console
+		 * printing, so boot consoles can share the same global printk
+		 * buffers.
+		 */
+		con->pbufs = &printk_shared_pbufs;
+	} else {
+		con->pbufs = kmalloc(sizeof(*con->pbufs), GFP_KERNEL);
+		if (!con->pbufs) {
+			con_printk(KERN_ERR, con, "failed to allocate printing buffer\n");
+			return false;
+		}
+	}
+
+	return true;
+}
+
+/**
+ * nbcon_init - Initialize the nbcon console specific data
+ * @con:	Console to initialize
+ *
+ * nbcon_alloc() *must* be called and succeed before this function
+ * is called.
+ *
+ * This function expects that the legacy @con->seq has been set.
+ */
+void nbcon_init(struct console *con)
+{
+	struct nbcon_state state = { };
+
+	/* nbcon_alloc() must have been called and successful! */
+	BUG_ON(!con->pbufs);
+
+	nbcon_seq_force(con, con->seq);
+	nbcon_state_set(con, &state);
+}
+
+/**
+ * nbcon_free - Free and cleanup the nbcon console specific data
+ * @con:	Console to free/cleanup nbcon data
+ */
+void nbcon_free(struct console *con)
+{
+	struct nbcon_state state = { };
+
+	nbcon_state_set(con, &state);
+
+	/* Boot consoles share global printk buffers. */
+	if (!(con->flags & CON_BOOT))
+		kfree(con->pbufs);
+
+	con->pbufs = NULL;
+}