printk: __pr_flush: use srcu console list iterator

Use srcu console list iteration for console list traversal.

Document why the console_lock is still necessary. Note that this
is a preparatory change for when console_lock no longer provides
synchronization for the console list.

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/20221116162152.193147-17-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 13bc960..abdbd3f 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3561,6 +3561,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
        struct console *c;
        u64 last_diff = 0;
        u64 printk_seq;
+       int cookie;
        u64 diff;
        u64 seq;
 
@@ -3571,9 +3572,15 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
        for (;;) {
                diff = 0;
 
+               /*
+                * Hold the console_lock to guarantee safe access to
+                * console->seq and to prevent changes to @console_suspended
+                * until all consoles have been processed.
+                */
                console_lock();
 
-               for_each_console(c) {
+               cookie = console_srcu_read_lock();
+               for_each_console_srcu(c) {
                        if (con && con != c)
                                continue;
                        if (!console_is_usable(c))
@@ -3582,6 +3589,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
                        if (printk_seq < seq)
                                diff += seq - printk_seq;
                }
+               console_srcu_read_unlock(cookie);
 
                /*
                 * If consoles are suspended, it cannot be expected that they