printk: console_flush_all: use srcu console list iterator

Guarantee safe iteration of the console list by using SRCU.

Note that in the case of a handover, the SRCU read lock is also
released. This is documented in the function description and as
comments in the code. It is a bit tricky, but this preserves the
lockdep lock ordering for the context handing over the
console_lock:

  console_lock()
  | mutex_acquire(&console_lock_dep_map)       <-- console lock
  |
  console_unlock()
  | console_flush_all()
  | | srcu_read_lock(&console_srcu)            <-- srcu lock
  | | console_emit_next_record()
  | | | console_lock_spinning_disable_and_check()
  | | | | srcu_read_unlock(&console_srcu)      <-- srcu unlock
  | | | | mutex_release(&console_lock_dep_map) <-- console unlock

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/20221116162152.193147-16-john.ogness@linutronix.de

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index cad65d34c96d3aa83a37a721479be1e5f24bf437..13bc960c7aa53b3a8f9a4b43d5fe2e98545e3c2b 100644 (file)
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1894,13 +1894,13 @@ static void console_lock_spinning_enable(void)
  * safe to start busy waiting for the lock. Second, it checks if
  * there is a busy waiter and passes the lock rights to her.
  *
- * Important: Callers lose the lock if there was a busy waiter.
- *     They must not touch items synchronized by console_lock
- *     in this case.
+ * Important: Callers lose both the console_lock and the SRCU read lock if
+ *     there was a busy waiter. They must not touch items synchronized by
+ *     console_lock or SRCU read lock in this case.
  *
  * Return: 1 if the lock rights were passed, 0 otherwise.
  */
-static int console_lock_spinning_disable_and_check(void)
+static int console_lock_spinning_disable_and_check(int cookie)
 {
        int waiter;
 
@@ -1919,6 +1919,12 @@ static int console_lock_spinning_disable_and_check(void)
 
        spin_release(&console_owner_dep_map, _THIS_IP_);
 
+       /*
+        * Preserve lockdep lock ordering. Release the SRCU read lock before
+        * releasing the console_lock.
+        */
+       console_srcu_read_unlock(cookie);
+
        /*
         * Hand off console_lock to waiter. The waiter will perform
         * the up(). After this, the waiter is the console_lock owner.
@@ -2402,7 +2408,7 @@ static ssize_t msg_print_ext_body(char *buf, size_t size,
                                  char *text, size_t text_len,
                                  struct dev_printk_info *dev_info) { return 0; }
 static void console_lock_spinning_enable(void) { }
-static int console_lock_spinning_disable_and_check(void) { return 0; }
+static int console_lock_spinning_disable_and_check(int cookie) { return 0; }
 static void call_console_driver(struct console *con, const char *text, size_t len,
                                char *dropped_text)
 {
@@ -2744,16 +2750,18 @@ static void __console_unlock(void)
  * DROPPED_TEXT_MAX. Otherwise @dropped_text must be NULL.
  *
  * @handover will be set to true if a printk waiter has taken over the
- * console_lock, in which case the caller is no longer holding the
- * console_lock. Otherwise it is set to false.
+ * console_lock, in which case the caller is no longer holding both the
+ * console_lock and the SRCU read lock. Otherwise it is set to false.
+ *
+ * @cookie is the cookie from the SRCU read lock.
  *
  * Returns false if the given console has no next record to print, otherwise
  * true.
  *
- * Requires the console_lock.
+ * Requires the console_lock and the SRCU read lock.
  */
 static bool console_emit_next_record(struct console *con, char *text, char *ext_text,
-                                    char *dropped_text, bool *handover)
+                                    char *dropped_text, bool *handover, int cookie)
 {
        static int panic_console_dropped;
        struct printk_info info;
@@ -2813,7 +2821,7 @@ static bool console_emit_next_record(struct console *con, char *text, char *ext_
 
        con->seq++;
 
-       *handover = console_lock_spinning_disable_and_check();
+       *handover = console_lock_spinning_disable_and_check(cookie);
        printk_safe_exit_irqrestore(flags);
 skip:
        return true;
@@ -2850,6 +2858,7 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
        bool any_usable = false;
        struct console *con;
        bool any_progress;
+       int cookie;
 
        *next_seq = 0;
        *handover = false;
@@ -2857,23 +2866,29 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
        do {
                any_progress = false;
 
-               for_each_console(con) {
+               cookie = console_srcu_read_lock();
+               for_each_console_srcu(con) {
                        bool progress;
 
                        if (!console_is_usable(con))
                                continue;
                        any_usable = true;
 
-                       if (con->flags & CON_EXTENDED) {
+                       if (console_srcu_read_flags(con) & CON_EXTENDED) {
                                /* Extended consoles do not print "dropped messages". */
                                progress = console_emit_next_record(con, &text[0],
                                                                    &ext_text[0], NULL,
-                                                                   handover);
+                                                                   handover, cookie);
                        } else {
                                progress = console_emit_next_record(con, &text[0],
                                                                    NULL, &dropped_text[0],
-                                                                   handover);
+                                                                   handover, cookie);
                        }
+
+                       /*
+                        * If a handover has occurred, the SRCU read lock
+                        * is already released.
+                        */
                        if (*handover)
                                return false;
 
@@ -2887,14 +2902,19 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 
                        /* Allow panic_cpu to take over the consoles safely. */
                        if (abandon_console_lock_in_panic())
-                               return false;
+                               goto abandon;
 
                        if (do_cond_resched)
                                cond_resched();
                }
+               console_srcu_read_unlock(cookie);
        } while (any_progress);
 
        return any_usable;
+
+abandon:
+       console_srcu_read_unlock(cookie);
+       return false;
 }
 
 /**