[PATCH RFC 07/11] dyndbg: use READ_ONCE()/WRITE_ONCE() to access descriptor flags

[Thomas Weißschuh]

The descriptor flags are accessed concurrently. Use
READ_ONCE()/WRITE_ONCE() to keep their contents consistent.

Signed-off-by: Thomas Weißschuh <linux@xxxxxxxxxxxxxx>

---
I am not entirely sure about this. We are only using single bits, so
torn reads/writes should not matter. But LKMM has no guidance on this as
far as I can see. Also for Rust this seems to be necessary.
---
 include/linux/dynamic_debug.h |  6 +++---
 lib/dynamic_debug.c           | 24 +++++++++++++-----------
 2 files changed, 16 insertions(+), 14 deletions(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index b1ff7c0e9a5f..6986142928ef 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -164,7 +164,7 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
 
 #define __dynamic_dump_stack(desc)				\
 {								\
-	if (desc.flags & _DPRINTK_FLAGS_INCL_STACK)		\
+	if (READ_ONCE(desc.flags) & _DPRINTK_FLAGS_INCL_STACK)	\
 		dump_stack();					\
 }
 
@@ -207,10 +207,10 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
 
 #ifdef DEBUG
 #define DYNAMIC_DEBUG_BRANCH(descriptor) \
-	likely(descriptor.flags & _DPRINTK_FLAGS_PRINT)
+	likely(READ_ONCE(descriptor.flags) & _DPRINTK_FLAGS_PRINT)
 #else
 #define DYNAMIC_DEBUG_BRANCH(descriptor) \
-	unlikely(descriptor.flags & _DPRINTK_FLAGS_PRINT)
+	unlikely(READ_ONCE(descriptor.flags) & _DPRINTK_FLAGS_PRINT)
 #endif
 
 #endif /* CONFIG_JUMP_LABEL */
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 18a71a9108d3..6dfd125fa6be 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -244,7 +244,7 @@ static int ddebug_change(const struct ddebug_query *query,
 
 			nfound++;
 
-			newflags = (dp->flags & modifiers->mask) | modifiers->flags;
+			newflags = (READ_ONCE(dp->flags) & modifiers->mask) | modifiers->flags;
 			if (newflags == dp->flags)
 				continue;
 #ifdef CONFIG_JUMP_LABEL
@@ -260,7 +260,7 @@ static int ddebug_change(const struct ddebug_query *query,
 				  dt->mod_name, dp->function,
 				  ddebug_describe_flags(dp->flags, &fbuf),
 				  ddebug_describe_flags(newflags, &nbuf));
-			dp->flags = newflags;
+			WRITE_ONCE(dp->flags, newflags);
 		}
 	}
 	mutex_unlock(&ddebug_lock);
@@ -822,12 +822,12 @@ static int remaining(int wrote)
 	return 0;
 }
 
-static char *__dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
+static char *__dynamic_emit_prefix(const struct _ddebug *desc, u8 flags, char *buf)
 {
 	int pos_after_tid;
 	int pos = 0;
 
-	if (desc->flags & _DPRINTK_FLAGS_INCL_TID) {
+	if (flags & _DPRINTK_FLAGS_INCL_TID) {
 		if (in_interrupt())
 			pos += snprintf(buf + pos, remaining(pos), "<intr> ");
 		else
@@ -835,16 +835,16 @@ static char *__dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
 					task_pid_vnr(current));
 	}
 	pos_after_tid = pos;
-	if (desc->flags & _DPRINTK_FLAGS_INCL_MODNAME)
+	if (flags & _DPRINTK_FLAGS_INCL_MODNAME)
 		pos += snprintf(buf + pos, remaining(pos), "%s:",
 				desc->modname);
-	if (desc->flags & _DPRINTK_FLAGS_INCL_FUNCNAME)
+	if (flags & _DPRINTK_FLAGS_INCL_FUNCNAME)
 		pos += snprintf(buf + pos, remaining(pos), "%s:",
 				desc->function);
-	if (desc->flags & _DPRINTK_FLAGS_INCL_SOURCENAME)
+	if (flags & _DPRINTK_FLAGS_INCL_SOURCENAME)
 		pos += snprintf(buf + pos, remaining(pos), "%s:",
 				trim_prefix(desc->filename));
-	if (desc->flags & _DPRINTK_FLAGS_INCL_LINENO)
+	if (flags & _DPRINTK_FLAGS_INCL_LINENO)
 		pos += snprintf(buf + pos, remaining(pos), "%d:",
 				desc->lineno);
 	if (pos - pos_after_tid)
@@ -857,8 +857,10 @@ static char *__dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
 
 static inline char *dynamic_emit_prefix(struct _ddebug *desc, char *buf)
 {
-	if (unlikely(desc->flags & _DPRINTK_FLAGS_INCL_ANY))
-		return __dynamic_emit_prefix(desc, buf);
+	u8 flags = READ_ONCE(desc->flags);
+
+	if (unlikely(flags & _DPRINTK_FLAGS_INCL_ANY))
+		return __dynamic_emit_prefix(desc, flags, buf);
 	return buf;
 }
 
@@ -1146,7 +1148,7 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
 	seq_printf(m, "%s:%u [%s]%s =%s \"",
 		   trim_prefix(dp->filename), dp->lineno,
 		   iter->table->mod_name, dp->function,
-		   ddebug_describe_flags(dp->flags, &flags));
+		   ddebug_describe_flags(READ_ONCE(dp->flags), &flags));
 	seq_escape_str(m, dp->format, ESCAPE_SPACE, "\t\r\n\"");
 	seq_putc(m, '"');
 

-- 
2.56.0