linux-kernel - [PATCH printk v3 39/40] printk: relieve console

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Message-Id: <20221107141638.3790965-40-john.ogness@linutronix.de>
Date:   Mon,  7 Nov 2022 15:22:37 +0106
From:   John Ogness <john.ogness@...utronix.de>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH printk v3 39/40] printk: relieve console_lock of list synchronization duties

The console_list_lock provides synchronization for console list and
console->flags updates. All call sites that were using the console_lock
for this synchronization have either switched to use the
console_list_lock or the SRCU list iterator.

Remove console_lock usage for console list updates and console->flags
updates.

Signed-off-by: John Ogness <john.ogness@...utronix.de>
---
 kernel/printk/printk.c | 36 ++++++++++++------------------------
 1 file changed, 12 insertions(+), 24 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index d74e6e609f7d..17765166ac42 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,8 +86,8 @@ EXPORT_SYMBOL(oops_in_progress);
 static DEFINE_MUTEX(console_mutex);
 
 /*
- * console_sem protects console_list and console->flags updates, and also
- * provides serialization for access to the entire console driver system.
+ * console_sem protects updates to console->seq and console_suspended,
+ * and also provides serialization for console printing.
  */
 static DEFINE_SEMAPHORE(console_sem);
 HLIST_HEAD(console_list);
@@ -2638,10 +2638,10 @@ static int console_cpu_notify(unsigned int cpu)
 }
 
 /**
- * console_lock - lock the console system for exclusive use.
+ * console_lock - block the console subsystem from printing
  *
- * Acquires a lock which guarantees that the caller has
- * exclusive access to the console system and console_list.
+ * Acquires a lock which guarantees that no consoles will
+ * be in or enter their write() callback.
  *
  * Can sleep, returns nothing.
  */
@@ -2658,10 +2658,10 @@ void console_lock(void)
 EXPORT_SYMBOL(console_lock);
 
 /**
- * console_trylock - try to lock the console system for exclusive use.
+ * console_trylock - try to block the console subsystem from printing
  *
- * Try to acquire a lock which guarantees that the caller has exclusive
- * access to the console system and console_list.
+ * Try to acquire a lock which guarantees that no consoles will
+ * be in or enter their write() callback.
  *
  * returns 1 on success, and 0 on failure to acquire the lock.
  */
@@ -2917,10 +2917,10 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
 }
 
 /**
- * console_unlock - unlock the console system
+ * console_unlock - unblock the console subsystem from printing
  *
- * Releases the console_lock which the caller holds on the console system
- * and the console driver list.
+ * Releases the console_lock which the caller holds to block printing of
+ * the console subsystem.
  *
  * While the console_lock was held, console output may have been buffered
  * by printk().  If this is the case, console_unlock(); emits
@@ -3107,9 +3107,7 @@ void console_stop(struct console *console)
 {
 	__pr_flush(console, 1000, true);
 	console_list_lock();
-	console_lock();
 	WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED);
-	console_unlock();
 	console_list_unlock();
 
 	/*
@@ -3125,9 +3123,7 @@ EXPORT_SYMBOL(console_stop);
 void console_start(struct console *console)
 {
 	console_list_lock();
-	console_lock();
 	WRITE_ONCE(console->flags, console->flags | CON_ENABLED);
-	console_unlock();
 	console_list_unlock();
 	__pr_flush(console, 1000, true);
 }
@@ -3344,7 +3340,6 @@ void register_console(struct console *newcon)
 	 * Put this console in the list - keep the
 	 * preferred driver at the head of the list.
 	 */
-	console_lock();
 	if (hlist_empty(&console_list)) {
 		/* Ensure CON_CONSDEV is always set for the head. */
 		newcon->flags |= CON_CONSDEV;
@@ -3358,7 +3353,6 @@ void register_console(struct console *newcon)
 	} else {
 		hlist_add_behind_rcu(&newcon->node, console_list.first);
 	}
-	console_unlock();
 
 	/*
 	 * No need to synchronize SRCU here! The caller does not rely
@@ -3406,15 +3400,11 @@ static int unregister_console_locked(struct console *console)
 	if (res > 0)
 		return 0;
 
-	console_lock();
-
 	/* Disable it unconditionally */
 	WRITE_ONCE(console->flags, console->flags & ~CON_ENABLED);
 
-	if (!console_is_registered_locked(console)) {
-		console_unlock();
+	if (!console_is_registered_locked(console))
 		return -ENODEV;
-	}
 
 	hlist_del_init_rcu(&console->node);
 
@@ -3430,8 +3420,6 @@ static int unregister_console_locked(struct console *console)
 	if (!hlist_empty(&console_list) && console->flags & CON_CONSDEV)
 		WRITE_ONCE(console_first()->flags, console_first()->flags | CON_CONSDEV);
 
-	console_unlock();
-
 	/*
 	 * Ensure that all SRCU list walks have completed. All contexts
 	 * must not be able to see this console in the list so that any
-- 
2.30.2