Merge tag 'printk-for-6.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux

Pull printk updates from Petr Mladek:

 - Allow creaing nbcon console drivers with an unsafe write_atomic()
   callback that can only be called by the final nbcon_atomic_flush_unsafe().
   Otherwise, the driver would rely on the kthread.

   It is going to be used as the-best-effort approach for an
   experimental nbcon netconsole driver, see

     https://lore.kernel.org/r/20251121-nbcon-v1-2-503d17b2b4af@debian.org

   Note that a safe .write_atomic() callback is supposed to work in NMI
   context. But some networking drivers are not safe even in IRQ
   context:

     https://lore.kernel.org/r/oc46gdpmmlly5o44obvmoatfqo5bhpgv7pabpvb6sjuqioymcg@gjsma3ghoz35

   In an ideal world, all networking drivers would be fixed first and
   the atomic flush would be blocked only in NMI context. But it brings
   the question how reliable networking drivers are when the system is
   in a bad state. They might block flushing more reliable serial
   consoles which are more suitable for serious debugging anyway.

 - Allow to use the last 4 bytes of the printk ring buffer.

 - Prevent queuing IRQ work and block printk kthreads when consoles are
   suspended. Otherwise, they create non-necessary churn or even block
   the suspend.

 - Release console_lock() between each record in the kthread used for
   legacy consoles on RT. It might significantly speed up the boot.

 - Release nbcon context between each record in the atomic flush. It
   prevents stalls of the related printk kthread after it has lost the
   ownership in the middle of a record

 - Add support for NBCON consoles into KDB

 - Add %ptsP modifier for printing struct timespec64 and use it where
   possible

 - Misc code clean up

* tag 'printk-for-6.19' of git://git.kernel.org/pub/scm/linux/kernel/git/printk/linux: (48 commits)
  printk: Use console_is_usable on console_unblank
  arch: um: kmsg_dump: Use console_is_usable
  drivers: serial: kgdboc: Drop checks for CON_ENABLED and CON_BOOT
  lib/vsprintf: Unify FORMAT_STATE_NUM handlers
  printk: Avoid irq_work for printk_deferred() on suspend
  printk: Avoid scheduling irq_work on suspend
  printk: Allow printk_trigger_flush() to flush all types
  tracing: Switch to use %ptSp
  scsi: snic: Switch to use %ptSp
  scsi: fnic: Switch to use %ptSp
  s390/dasd: Switch to use %ptSp
  ptp: ocp: Switch to use %ptSp
  pps: Switch to use %ptSp
  PCI: epf-test: Switch to use %ptSp
  net: dsa: sja1105: Switch to use %ptSp
  mmc: mmc_test: Switch to use %ptSp
  media: av7110: Switch to use %ptSp
  ipmi: Switch to use %ptSp
  igb: Switch to use %ptSp
  e1000e: Switch to use %ptSp
  ...

1 files changed, 53 insertions, 14 deletions

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 40198bffb7d0..56c8e3d031f4 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -411,6 +411,23 @@ static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
 	return to_blk_size(size) <= DATA_SIZE(data_ring) / 2;
 }
 
+/*
+ * Compare the current and requested logical position and decide
+ * whether more space is needed.
+ *
+ * Return false when @lpos_current is already at or beyond @lpos_target.
+ *
+ * Also return false when the difference between the positions is bigger
+ * than the size of the data buffer. It might happen only when the caller
+ * raced with another CPU(s) which already made and used the space.
+ */
+static bool need_more_space(struct prb_data_ring *data_ring,
+			    unsigned long lpos_current,
+			    unsigned long lpos_target)
+{
+	return lpos_target - lpos_current - 1 < DATA_SIZE(data_ring);
+}
+
 /* Query the state of a descriptor. */
 static enum desc_state get_desc_state(unsigned long id,
 				      unsigned long state_val)
@@ -577,7 +594,7 @@ static bool data_make_reusable(struct printk_ringbuffer *rb,
 	unsigned long id;
 
 	/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
-	while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
+	while (need_more_space(data_ring, lpos_begin, lpos_end)) {
 		blk = to_block(data_ring, lpos_begin);
 
 		/*
@@ -668,7 +685,7 @@ static bool data_push_tail(struct printk_ringbuffer *rb, unsigned long lpos)
 	 * sees the new tail lpos, any descriptor states that transitioned to
 	 * the reusable state must already be visible.
 	 */
-	while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
+	while (need_more_space(data_ring, tail_lpos, lpos)) {
 		/*
 		 * Make all descriptors reusable that are associated with
 		 * data blocks before @lpos.
@@ -999,6 +1016,17 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
 	return true;
 }
 
+static bool is_blk_wrapped(struct prb_data_ring *data_ring,
+			   unsigned long begin_lpos, unsigned long next_lpos)
+{
+	/*
+	 * Subtract one from next_lpos since it's not actually part of this data
+	 * block. This allows perfectly fitting records to not wrap.
+	 */
+	return DATA_WRAPS(data_ring, begin_lpos) !=
+	       DATA_WRAPS(data_ring, next_lpos - 1);
+}
+
 /* Determine the end of a data block. */
 static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
 				   unsigned long lpos, unsigned int size)
@@ -1010,7 +1038,7 @@ static unsigned long get_next_lpos(struct prb_data_ring *data_ring,
 	next_lpos = lpos + size;
 
 	/* First check if the data block does not wrap. */
-	if (DATA_WRAPS(data_ring, begin_lpos) == DATA_WRAPS(data_ring, next_lpos))
+	if (!is_blk_wrapped(data_ring, begin_lpos, next_lpos))
 		return next_lpos;
 
 	/* Wrapping data blocks store their data at the beginning. */
@@ -1087,7 +1115,7 @@ static char *data_alloc(struct printk_ringbuffer *rb, unsigned int size,
 	blk = to_block(data_ring, begin_lpos);
 	blk->id = id; /* LMM(data_alloc:B) */
 
-	if (DATA_WRAPS(data_ring, begin_lpos) != DATA_WRAPS(data_ring, next_lpos)) {
+	if (is_blk_wrapped(data_ring, begin_lpos, next_lpos)) {
 		/* Wrapping data blocks store their data at the beginning. */
 		blk = to_block(data_ring, 0);
 
@@ -1131,14 +1159,21 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
 		return NULL;
 
 	/* Keep track if @blk_lpos was a wrapping data block. */
-	wrapped = (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, blk_lpos->next));
+	wrapped = is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next);
 
 	size = to_blk_size(size);
 
 	next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);
 
-	/* If the data block does not increase, there is nothing to do. */
-	if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
+	/*
+	 * Use the current data block when the size does not increase, i.e.
+	 * when @head_lpos is already able to accommodate the new @next_lpos.
+	 *
+	 * Note that need_more_space() could never return false here because
+	 * the difference between the positions was bigger than the data
+	 * buffer size. The data block is reopened and can't get reused.
+	 */
+	if (!need_more_space(data_ring, head_lpos, next_lpos)) {
 		if (wrapped)
 			blk = to_block(data_ring, 0);
 		else
@@ -1167,7 +1202,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
 
 	blk = to_block(data_ring, blk_lpos->begin);
 
-	if (DATA_WRAPS(data_ring, blk_lpos->begin) != DATA_WRAPS(data_ring, next_lpos)) {
+	if (is_blk_wrapped(data_ring, blk_lpos->begin, next_lpos)) {
 		struct prb_data_block *old_blk = blk;
 
 		/* Wrapping data blocks store their data at the beginning. */
@@ -1203,7 +1238,7 @@ static unsigned int space_used(struct prb_data_ring *data_ring,
 	if (BLK_DATALESS(blk_lpos))
 		return 0;
 
-	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next)) {
+	if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next)) {
 		/* Data block does not wrap. */
 		return (DATA_INDEX(data_ring, blk_lpos->next) -
 			DATA_INDEX(data_ring, blk_lpos->begin));
@@ -1249,15 +1284,15 @@ static const char *get_data(struct prb_data_ring *data_ring,
 		return NULL;
 	}
 
-	/* Regular data block: @begin less than @next and in same wrap. */
-	if (DATA_WRAPS(data_ring, blk_lpos->begin) == DATA_WRAPS(data_ring, blk_lpos->next) &&
-	    blk_lpos->begin < blk_lpos->next) {
+	/* Regular data block: @begin and @next in the same wrap. */
+	if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next)) {
 		db = to_block(data_ring, blk_lpos->begin);
 		*data_size = blk_lpos->next - blk_lpos->begin;
 
 	/* Wrapping data block: @begin is one wrap behind @next. */
-	} else if (DATA_WRAPS(data_ring, blk_lpos->begin + DATA_SIZE(data_ring)) ==
-		   DATA_WRAPS(data_ring, blk_lpos->next)) {
+	} else if (!is_blk_wrapped(data_ring,
+				   blk_lpos->begin + DATA_SIZE(data_ring),
+				   blk_lpos->next)) {
 		db = to_block(data_ring, 0);
 		*data_size = DATA_INDEX(data_ring, blk_lpos->next);
 
@@ -1267,6 +1302,10 @@ static const char *get_data(struct prb_data_ring *data_ring,
 		return NULL;
 	}
 
+	/* Sanity check. Data-less blocks were handled earlier. */
+	if (WARN_ON_ONCE(!data_check_size(data_ring, *data_size) || !*data_size))
+		return NULL;
+
 	/* A valid data block will always be aligned to the ID size. */
 	if (WARN_ON_ONCE(blk_lpos->begin != ALIGN(blk_lpos->begin, sizeof(db->id))) ||
 	    WARN_ON_ONCE(blk_lpos->next != ALIGN(blk_lpos->next, sizeof(db->id)))) {