kdb: use srcu console list iterator

Guarantee safe iteration of the console list by using SRCU.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Aaron Tomlin <atomlin@atomlin.com>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20221116162152.193147-15-john.ogness@linutronix.de

diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index 67d3c48a15222212f4c8002887059d938d4ea042..5c7e9ba7cd6b25a7309752f6e02ab42ceac35dbb 100644 (file)
--- 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)