printk: ringbuffer: change representation of states

Rather than deriving the state by evaluating bits within the flags
area of the state variable, assign the states explicit values and
set those values in the flags area. Introduce macros to make it
simple to read and write state values for the state variable.

Although the functionality is preserved, the binary representation
for the states is changed.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200914123354.832-5-john.ogness@linutronix.de

Documentation/admin-guide/kdump/gdbmacros.txt

@@ -295,9 +295,12 @@ document dump_record
 end
 
 define dmesg
-	set var $desc_committed = 1UL << ((sizeof(long) * 8) - 1)
-	set var $flags_mask = 3UL << ((sizeof(long) * 8) - 2)
-	set var $id_mask = ~$flags_mask
+	# definitions from kernel/printk/printk_ringbuffer.h
+	set var $desc_committed = 1
+	set var $desc_sv_bits = sizeof(long) * 8
+	set var $desc_flags_shift = $desc_sv_bits - 2
+	set var $desc_flags_mask = 3 << $desc_flags_shift
+	set var $id_mask = ~$desc_flags_mask
 
 	set var $desc_count = 1U << prb->desc_ring.count_bits
 	set var $prev_flags = 0
@@ -309,7 +312,8 @@ define dmesg
 		set var $desc = &prb->desc_ring.descs[$id % $desc_count]
 
 		# skip non-committed record
-		if (($desc->state_var.counter & $flags_mask) == $desc_committed)
+		set var $state = 3 & ($desc->state_var.counter >> $desc_flags_shift)
+		if ($state == $desc_committed)
 			dump_record $desc $prev_flags
 			set var $prev_flags = $desc->info.flags
 		end

kernel/printk/printk_ringbuffer.c

@@ -348,14 +348,6 @@ static bool data_check_size(struct prb_data_ring *data_ring, unsigned int size)
 	return true;
 }
 
-/* The possible responses of a descriptor state-query. */
-enum desc_state {
-	desc_miss,	/* ID mismatch */
-	desc_reserved,	/* reserved, in use by writer */
-	desc_committed, /* committed, writer is done */
-	desc_reusable,	/* free, not yet used by any writer */
-};
-
 /* Query the state of a descriptor. */
 static enum desc_state get_desc_state(unsigned long id,
 				      unsigned long state_val)
@@ -363,13 +355,7 @@ static enum desc_state get_desc_state(unsigned long id,
 	if (id != DESC_ID(state_val))
 		return desc_miss;
 
-	if (state_val & DESC_REUSE_MASK)
-		return desc_reusable;
-
-	if (state_val & DESC_COMMITTED_MASK)
-		return desc_committed;
-
-	return desc_reserved;
+	return DESC_STATE(state_val);
 }
 
 /*
@@ -484,8 +470,8 @@ static enum desc_state desc_read(struct prb_desc_ring *desc_ring,
 static void desc_make_reusable(struct prb_desc_ring *desc_ring,
 			       unsigned long id)
 {
-	unsigned long val_committed = id | DESC_COMMITTED_MASK;
-	unsigned long val_reusable = val_committed | DESC_REUSE_MASK;
+	unsigned long val_committed = DESC_SV(id, desc_committed);
+	unsigned long val_reusable = DESC_SV(id, desc_reusable);
 	struct prb_desc *desc = to_desc(desc_ring, id);
 	atomic_long_t *state_var = &desc->state_var;
 
@@ -921,7 +907,7 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
 	 */
 	prev_state_val = atomic_long_read(&desc->state_var); /* LMM(desc_reserve:E) */
 	if (prev_state_val &&
-	    prev_state_val != (id_prev_wrap | DESC_COMMITTED_MASK | DESC_REUSE_MASK)) {
+	    get_desc_state(id_prev_wrap, prev_state_val) != desc_reusable) {
 		WARN_ON_ONCE(1);
 		return false;
 	}
@@ -935,7 +921,7 @@ static bool desc_reserve(struct printk_ringbuffer *rb, unsigned long *id_out)
 	 * This pairs with desc_read:D.
 	 */
 	if (!atomic_long_try_cmpxchg(&desc->state_var, &prev_state_val,
-				     id | 0)) { /* LMM(desc_reserve:F) */
+			DESC_SV(id, desc_reserved))) { /* LMM(desc_reserve:F) */
 		WARN_ON_ONCE(1);
 		return false;
 	}
@@ -1254,7 +1240,7 @@ void prb_commit(struct prb_reserved_entry *e)
 {
 	struct prb_desc_ring *desc_ring = &e->rb->desc_ring;
 	struct prb_desc *d = to_desc(desc_ring, e->id);
-	unsigned long prev_state_val = e->id | 0;
+	unsigned long prev_state_val = DESC_SV(e->id, desc_reserved);
 
 	/* Now the writer has finished all writing: LMM(prb_commit:A) */
 
@@ -1267,7 +1253,7 @@ void prb_commit(struct prb_reserved_entry *e)
 	 * this. This pairs with desc_read:B.
 	 */
 	if (!atomic_long_try_cmpxchg(&d->state_var, &prev_state_val,
-				     e->id | DESC_COMMITTED_MASK)) { /* LMM(prb_commit:B) */
+			DESC_SV(e->id, desc_committed))) { /* LMM(prb_commit:B) */
 		WARN_ON_ONCE(1);
 	}
 

kernel/printk/printk_ringbuffer.h

@@ -112,12 +112,21 @@ struct prb_reserved_entry {
 	unsigned int			text_space;
 };
 
+/* The possible responses of a descriptor state-query. */
+enum desc_state {
+	desc_miss	=  -1,	/* ID mismatch (pseudo state) */
+	desc_reserved	= 0x0,	/* reserved, in use by writer */
+	desc_committed	= 0x1,	/* committed by writer */
+	desc_reusable	= 0x3,	/* free, not yet used by any writer */
+};
+
 #define _DATA_SIZE(sz_bits)	(1UL << (sz_bits))
 #define _DESCS_COUNT(ct_bits)	(1U << (ct_bits))
 #define DESC_SV_BITS		(sizeof(unsigned long) * 8)
-#define DESC_COMMITTED_MASK		(1UL << (DESC_SV_BITS - 1))
-#define DESC_REUSE_MASK			(1UL << (DESC_SV_BITS - 2))
-#define DESC_FLAGS_MASK			(DESC_COMMITTED_MASK | DESC_REUSE_MASK)
+#define DESC_FLAGS_SHIFT	(DESC_SV_BITS - 2)
+#define DESC_FLAGS_MASK		(3UL << DESC_FLAGS_SHIFT)
+#define DESC_STATE(sv)		(3UL & (sv >> DESC_FLAGS_SHIFT))
+#define DESC_SV(id, state)	(((unsigned long)state << DESC_FLAGS_SHIFT) | id)
 #define DESC_ID_MASK		(~DESC_FLAGS_MASK)
 #define DESC_ID(sv)		((sv) & DESC_ID_MASK)
 #define FAILED_LPOS		0x1
@@ -213,7 +222,7 @@ struct prb_reserved_entry {
  */
 #define BLK0_LPOS(sz_bits)	(-(_DATA_SIZE(sz_bits)))
 #define DESC0_ID(ct_bits)	DESC_ID(-(_DESCS_COUNT(ct_bits) + 1))
-#define DESC0_SV(ct_bits)	(DESC_COMMITTED_MASK | DESC_REUSE_MASK | DESC0_ID(ct_bits))
+#define DESC0_SV(ct_bits)	DESC_SV(DESC0_ID(ct_bits), desc_reusable)
 
 /*
  * Define a ringbuffer with an external text data buffer. The same as

scripts/gdb/linux/dmesg.py

@@ -78,10 +78,10 @@ class LxDmesg(gdb.Command):
         len_off = off + printk_info_type.get_type()['text_len'].bitpos // 8
 
         # definitions from kernel/printk/printk_ringbuffer.h
+        desc_committed = 1
         desc_sv_bits = utils.get_long_type().sizeof * 8
-        desc_committed_mask = 1 << (desc_sv_bits - 1)
-        desc_reuse_mask = 1 << (desc_sv_bits - 2)
-        desc_flags_mask = desc_committed_mask | desc_reuse_mask
+        desc_flags_shift = desc_sv_bits - 2
+        desc_flags_mask = 3 << desc_flags_shift
         desc_id_mask = ~desc_flags_mask
 
         # read in tail and head descriptor ids
@@ -96,8 +96,9 @@ class LxDmesg(gdb.Command):
             desc_off = desc_sz * ind
 
             # skip non-committed record
-            state = utils.read_u64(descs, desc_off + sv_off + counter_off) & desc_flags_mask
-            if state != desc_committed_mask:
+            state = 3 & (utils.read_u64(descs, desc_off + sv_off +
+                                        counter_off) >> desc_flags_shift)
+            if state != desc_committed:
                 if did == head_id:
                     break
                 did = (did + 1) & desc_id_mask