[PATCH printk v5 14/40] kdb: use srcu console list iterator

[John Ogness]

Guarantee safe iteration of the console list by using SRCU.

Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Reviewed-by: Aaron Tomlin <atomlin@xxxxxxxxxxx>
---
 kernel/debug/kdb/kdb_io.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 67d3c48a1522..5c7e9ba7cd6b 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -545,6 +545,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
 {
 	struct console *c;
 	const char *cp;
+	int cookie;
 	int len;
 
 	if (msg_len == 0)
@@ -558,8 +559,20 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		cp++;
 	}
 
-	for_each_console(c) {
-		if (!(c->flags & CON_ENABLED))
+	/*
+	 * The console_srcu_read_lock() only provides safe console list
+	 * traversal. The use of the ->write() callback relies on all other
+	 * CPUs being stopped at the moment and console drivers being able to
+	 * handle reentrance when @oops_in_progress is set.
+	 *
+	 * There is no guarantee that every console driver can handle
+	 * reentrance in this way; the developer deploying the debugger
+	 * is responsible for ensuring that the console drivers they
+	 * have selected handle reentrance appropriately.
+	 */
+	cookie = console_srcu_read_lock();
+	for_each_console_srcu(c) {
+		if (!(console_srcu_read_flags(c) & CON_ENABLED))
 			continue;
 		if (c == dbg_io_ops->cons)
 			continue;
@@ -577,6 +590,7 @@ static void kdb_msg_write(const char *msg, int msg_len)
 		--oops_in_progress;
 		touch_nmi_watchdog();
 	}
+	console_srcu_read_unlock(cookie);
 }
 
 int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
-- 
2.30.2