1 files changed, 266 insertions, 49 deletions

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index fde338606ce8..f5a8bb606fe5 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -6,6 +6,7 @@
 #include <linux/errno.h>
 #include <linux/bug.h>
 #include "printk_ringbuffer.h"
+#include "internal.h"
 
 /**
  * DOC: printk_ringbuffer overview
@@ -303,6 +304,9 @@
  *
  *   desc_push_tail:B / desc_reserve:D
  *     set descriptor reusable (state), then push descriptor tail (id)
+ *
+ *   desc_update_last_finalized:A / desc_last_finalized_seq:A
+ *     store finalized record, then set new highest finalized sequence number
  */
 
 #define DATA_SIZE(data_ring)		_DATA_SIZE((data_ring)->size_bits)
@@ -1442,19 +1446,117 @@ fail_reopen:
 }
 
 /*
+ * @last_finalized_seq value guarantees that all records up to and including
+ * this sequence number are finalized and can be read. The only exception are
+ * too old records which have already been overwritten.
+ *
+ * It is also guaranteed that @last_finalized_seq only increases.
+ *
+ * Be aware that finalized records following non-finalized records are not
+ * reported because they are not yet available to the reader. For example,
+ * a new record stored via printk() will not be available to a printer if
+ * it follows a record that has not been finalized yet. However, once that
+ * non-finalized record becomes finalized, @last_finalized_seq will be
+ * appropriately updated and the full set of finalized records will be
+ * available to the printer. And since each printk() caller will either
+ * directly print or trigger deferred printing of all available unprinted
+ * records, all printk() messages will get printed.
+ */
+static u64 desc_last_finalized_seq(struct printk_ringbuffer *rb)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	unsigned long ulseq;
+
+	/*
+	 * Guarantee the sequence number is loaded before loading the
+	 * associated record in order to guarantee that the record can be
+	 * seen by this CPU. This pairs with desc_update_last_finalized:A.
+	 */
+	ulseq = atomic_long_read_acquire(&desc_ring->last_finalized_seq
+					); /* LMM(desc_last_finalized_seq:A) */
+
+	return __ulseq_to_u64seq(rb, ulseq);
+}
+
+static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
+			    struct printk_record *r, unsigned int *line_count);
+
+/*
+ * Check if there are records directly following @last_finalized_seq that are
+ * finalized. If so, update @last_finalized_seq to the latest of these
+ * records. It is not allowed to skip over records that are not yet finalized.
+ */
+static void desc_update_last_finalized(struct printk_ringbuffer *rb)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	u64 old_seq = desc_last_finalized_seq(rb);
+	unsigned long oldval;
+	unsigned long newval;
+	u64 finalized_seq;
+	u64 try_seq;
+
+try_again:
+	finalized_seq = old_seq;
+	try_seq = finalized_seq + 1;
+
+	/* Try to find later finalized records. */
+	while (_prb_read_valid(rb, &try_seq, NULL, NULL)) {
+		finalized_seq = try_seq;
+		try_seq++;
+	}
+
+	/* No update needed if no later finalized record was found. */
+	if (finalized_seq == old_seq)
+		return;
+
+	oldval = __u64seq_to_ulseq(old_seq);
+	newval = __u64seq_to_ulseq(finalized_seq);
+
+	/*
+	 * Set the sequence number of a later finalized record that has been
+	 * seen.
+	 *
+	 * Guarantee the record data is visible to other CPUs before storing
+	 * its sequence number. This pairs with desc_last_finalized_seq:A.
+	 *
+	 * Memory barrier involvement:
+	 *
+	 * If desc_last_finalized_seq:A reads from
+	 * desc_update_last_finalized:A, then desc_read:A reads from
+	 * _prb_commit:B.
+	 *
+	 * Relies on:
+	 *
+	 * RELEASE from _prb_commit:B to desc_update_last_finalized:A
+	 *    matching
+	 * ACQUIRE from desc_last_finalized_seq:A to desc_read:A
+	 *
+	 * Note: _prb_commit:B and desc_update_last_finalized:A can be
+	 *       different CPUs. However, the desc_update_last_finalized:A
+	 *       CPU (which performs the release) must have previously seen
+	 *       _prb_commit:B.
+	 */
+	if (!atomic_long_try_cmpxchg_release(&desc_ring->last_finalized_seq,
+				&oldval, newval)) { /* LMM(desc_update_last_finalized:A) */
+		old_seq = __ulseq_to_u64seq(rb, oldval);
+		goto try_again;
+	}
+}
+
+/*
  * Attempt to finalize a specified descriptor. If this fails, the descriptor
  * is either already final or it will finalize itself when the writer commits.
  */
-static void desc_make_final(struct prb_desc_ring *desc_ring, unsigned long id)
+static void desc_make_final(struct printk_ringbuffer *rb, unsigned long id)
 {
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
 	unsigned long prev_state_val = DESC_SV(id, desc_committed);
 	struct prb_desc *d = to_desc(desc_ring, id);
 
-	atomic_long_cmpxchg_relaxed(&d->state_var, prev_state_val,
-			DESC_SV(id, desc_finalized)); /* LMM(desc_make_final:A) */
-
-	/* Best effort to remember the last finalized @id. */
-	atomic_long_set(&desc_ring->last_finalized_id, id);
+	if (atomic_long_try_cmpxchg_relaxed(&d->state_var, &prev_state_val,
+			DESC_SV(id, desc_finalized))) { /* LMM(desc_make_final:A) */
+		desc_update_last_finalized(rb);
+	}
 }
 
 /**
@@ -1550,7 +1652,7 @@ bool prb_reserve(struct prb_reserved_entry *e, struct printk_ringbuffer *rb,
 	 * readers. (For seq==0 there is no previous descriptor.)
 	 */
 	if (info->seq > 0)
-		desc_make_final(desc_ring, DESC_ID(id - 1));
+		desc_make_final(rb, DESC_ID(id - 1));
 
 	r->text_buf = data_alloc(rb, r->text_buf_size, &d->text_blk_lpos, id);
 	/* If text data allocation fails, a data-less record is committed. */
@@ -1643,7 +1745,7 @@ void prb_commit(struct prb_reserved_entry *e)
 	 */
 	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_commit:A) */
 	if (head_id != e->id)
-		desc_make_final(desc_ring, e->id);
+		desc_make_final(e->rb, e->id);
 }
 
 /**
@@ -1663,12 +1765,9 @@ void prb_commit(struct prb_reserved_entry *e)
  */
 void prb_final_commit(struct prb_reserved_entry *e)
 {
-	struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
-
 	_prb_commit(e, desc_finalized);
 
-	/* Best effort to remember the last finalized @id. */
-	atomic_long_set(&desc_ring->last_finalized_id, e->id);
+	desc_update_last_finalized(e->rb);
 }
 
 /*
@@ -1832,7 +1931,7 @@ static int prb_read(struct printk_ringbuffer *rb, u64 seq,
 }
 
 /* Get the sequence number of the tail descriptor. */
-static u64 prb_first_seq(struct printk_ringbuffer *rb)
+u64 prb_first_seq(struct printk_ringbuffer *rb)
 {
 	struct prb_desc_ring *desc_ring = &rb->desc_ring;
 	enum desc_state d_state;
@@ -1875,12 +1974,123 @@ static u64 prb_first_seq(struct printk_ringbuffer *rb)
 	return seq;
 }
 
+/**
+ * prb_next_reserve_seq() - Get the sequence number after the most recently
+ *                  reserved record.
+ *
+ * @rb:  The ringbuffer to get the sequence number from.
+ *
+ * This is the public function available to readers to see what sequence
+ * number will be assigned to the next reserved record.
+ *
+ * Note that depending on the situation, this value can be equal to or
+ * higher than the sequence number returned by prb_next_seq().
+ *
+ * Context: Any context.
+ * Return: The sequence number that will be assigned to the next record
+ *         reserved.
+ */
+u64 prb_next_reserve_seq(struct printk_ringbuffer *rb)
+{
+	struct prb_desc_ring *desc_ring = &rb->desc_ring;
+	unsigned long last_finalized_id;
+	atomic_long_t *state_var;
+	u64 last_finalized_seq;
+	unsigned long head_id;
+	struct prb_desc desc;
+	unsigned long diff;
+	struct prb_desc *d;
+	int err;
+
+	/*
+	 * It may not be possible to read a sequence number for @head_id.
+	 * So the ID of @last_finailzed_seq is used to calculate what the
+	 * sequence number of @head_id will be.
+	 */
+
+try_again:
+	last_finalized_seq = desc_last_finalized_seq(rb);
+
+	/*
+	 * @head_id is loaded after @last_finalized_seq to ensure that
+	 * it points to the record with @last_finalized_seq or newer.
+	 *
+	 * Memory barrier involvement:
+	 *
+	 * If desc_last_finalized_seq:A reads from
+	 * desc_update_last_finalized:A, then
+	 * prb_next_reserve_seq:A reads from desc_reserve:D.
+	 *
+	 * Relies on:
+	 *
+	 * RELEASE from desc_reserve:D to desc_update_last_finalized:A
+	 *    matching
+	 * ACQUIRE from desc_last_finalized_seq:A to prb_next_reserve_seq:A
+	 *
+	 * Note: desc_reserve:D and desc_update_last_finalized:A can be
+	 *       different CPUs. However, the desc_update_last_finalized:A CPU
+	 *       (which performs the release) must have previously seen
+	 *       desc_read:C, which implies desc_reserve:D can be seen.
+	 */
+	head_id = atomic_long_read(&desc_ring->head_id); /* LMM(prb_next_reserve_seq:A) */
+
+	d = to_desc(desc_ring, last_finalized_seq);
+	state_var = &d->state_var;
+
+	/* Extract the ID, used to specify the descriptor to read. */
+	last_finalized_id = DESC_ID(atomic_long_read(state_var));
+
+	/* Ensure @last_finalized_id is correct. */
+	err = desc_read_finalized_seq(desc_ring, last_finalized_id, last_finalized_seq, &desc);
+
+	if (err == -EINVAL) {
+		if (last_finalized_seq == 0) {
+			/*
+			 * No record has been finalized or even reserved yet.
+			 *
+			 * The @head_id is initialized such that the first
+			 * increment will yield the first record (seq=0).
+			 * Handle it separately to avoid a negative @diff
+			 * below.
+			 */
+			if (head_id == DESC0_ID(desc_ring->count_bits))
+				return 0;
+
+			/*
+			 * One or more descriptors are already reserved. Use
+			 * the descriptor ID of the first one (@seq=0) for
+			 * the @diff below.
+			 */
+			last_finalized_id = DESC0_ID(desc_ring->count_bits) + 1;
+		} else {
+			/* Record must have been overwritten. Try again. */
+			goto try_again;
+		}
+	}
+
+	/* Diff of known descriptor IDs to compute related sequence numbers. */
+	diff = head_id - last_finalized_id;
+
+	/*
+	 * @head_id points to the most recently reserved record, but this
+	 * function returns the sequence number that will be assigned to the
+	 * next (not yet reserved) record. Thus +1 is needed.
+	 */
+	return (last_finalized_seq + diff + 1);
+}
+
 /*
- * Non-blocking read of a record. Updates @seq to the last finalized record
- * (which may have no data available).
+ * Non-blocking read of a record.
+ *
+ * On success @seq is updated to the record that was read and (if provided)
+ * @r and @line_count will contain the read/calculated data.
+ *
+ * On failure @seq is updated to a record that is not yet available to the
+ * reader, but it will be the next record available to the reader.
  *
- * See the description of prb_read_valid() and prb_read_valid_info()
- * for details.
+ * Note: When the current CPU is in panic, this function will skip over any
+ *       non-existent/non-finalized records in order to allow the panic CPU
+ *       to print any and all records that have been finalized.
  */
 static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			    struct printk_record *r, unsigned int *line_count)
@@ -1899,12 +2109,32 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
 			*seq = tail_seq;
 
 		} else if (err == -ENOENT) {
-			/* Record exists, but no data available. Skip. */
+			/* Record exists, but the data was lost. Skip. */
 			(*seq)++;
 
 		} else {
-			/* Non-existent/non-finalized record. Must stop. */
-			return false;
+			/*
+			 * Non-existent/non-finalized record. Must stop.
+			 *
+			 * For panic situations it cannot be expected that
+			 * non-finalized records will become finalized. But
+			 * there may be other finalized records beyond that
+			 * need to be printed for a panic situation. If this
+			 * is the panic CPU, skip this
+			 * non-existent/non-finalized record unless it is
+			 * at or beyond the head, in which case it is not
+			 * possible to continue.
+			 *
+			 * Note that new messages printed on panic CPU are
+			 * finalized when we are here. The only exception
+			 * might be the last message without trailing newline.
+			 * But it would have the sequence number returned
+			 * by "prb_next_reserve_seq() - 1".
+			 */
+			if (this_cpu_in_panic() && ((*seq + 1) < prb_next_reserve_seq(rb)))
+				(*seq)++;
+			else
+				return false;
 		}
 	}
 
@@ -1932,7 +2162,7 @@ static bool _prb_read_valid(struct printk_ringbuffer *rb, u64 *seq,
  * On success, the reader must check r->info.seq to see which record was
  * actually read. This allows the reader to detect dropped records.
  *
- * Failure means @seq refers to a not yet written record.
+ * Failure means @seq refers to a record not yet available to the reader.
  */
 bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
 		    struct printk_record *r)
@@ -1962,7 +2192,7 @@ bool prb_read_valid(struct printk_ringbuffer *rb, u64 seq,
  * On success, the reader must check info->seq to see which record meta data
  * was actually read. This allows the reader to detect dropped records.
  *
- * Failure means @seq refers to a not yet written record.
+ * Failure means @seq refers to a record not yet available to the reader.
  */
 bool prb_read_valid_info(struct printk_ringbuffer *rb, u64 seq,
 			 struct printk_info *info, unsigned int *line_count)
@@ -2008,7 +2238,9 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
  * newest sequence number available to readers will be.
  *
  * This provides readers a sequence number to jump to if all currently
- * available records should be skipped.
+ * available records should be skipped. It is guaranteed that all records
+ * previous to the returned value have been finalized and are (or were)
+ * available to the reader.
  *
  * Context: Any context.
  * Return: The sequence number of the next newest (not yet available) record
@@ -2016,34 +2248,19 @@ u64 prb_first_valid_seq(struct printk_ringbuffer *rb)
  */
 u64 prb_next_seq(struct printk_ringbuffer *rb)
 {
-	struct prb_desc_ring *desc_ring = &rb->desc_ring;
-	enum desc_state d_state;
-	unsigned long id;
 	u64 seq;
 
-	/* Check if the cached @id still points to a valid @seq. */
-	id = atomic_long_read(&desc_ring->last_finalized_id);
-	d_state = desc_read(desc_ring, id, NULL, &seq, NULL);
+	seq = desc_last_finalized_seq(rb);
 
-	if (d_state == desc_finalized || d_state == desc_reusable) {
-		/*
-		 * Begin searching after the last finalized record.
-		 *
-		 * On 0, the search must begin at 0 because of hack#2
-		 * of the bootstrapping phase it is not known if a
-		 * record at index 0 exists.
-		 */
-		if (seq != 0)
-			seq++;
-	} else {
-		/*
-		 * The information about the last finalized sequence number
-		 * has gone. It should happen only when there is a flood of
-		 * new messages and the ringbuffer is rapidly recycled.
-		 * Give up and start from the beginning.
-		 */
-		seq = 0;
-	}
+	/*
+	 * Begin searching after the last finalized record.
+	 *
+	 * On 0, the search must begin at 0 because of hack#2
+	 * of the bootstrapping phase it is not known if a
+	 * record at index 0 exists.
+	 */
+	if (seq != 0)
+		seq++;
 
 	/*
 	 * The information about the last finalized @seq might be inaccurate.
@@ -2085,7 +2302,7 @@ void prb_init(struct printk_ringbuffer *rb,
 	rb->desc_ring.infos = infos;
 	atomic_long_set(&rb->desc_ring.head_id, DESC0_ID(descbits));
 	atomic_long_set(&rb->desc_ring.tail_id, DESC0_ID(descbits));
-	atomic_long_set(&rb->desc_ring.last_finalized_id, DESC0_ID(descbits));
+	atomic_long_set(&rb->desc_ring.last_finalized_seq, 0);
 
 	rb->text_data_ring.size_bits = textbits;
 	rb->text_data_ring.data = text_buf;