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Message-Id: <20221114162932.141883-15-john.ogness@linutronix.de>
Date: Mon, 14 Nov 2022 17:35:07 +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 v4 14/39] 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@...utronix.de>
Reviewed-by: Petr Mladek <pmladek@...e.com>
---
kernel/printk/printk.c | 50 +++++++++++++++++++++++++++++-------------
1 file changed, 35 insertions(+), 15 deletions(-)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 41c90b768c1c..b37ca16abbd0 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1893,13 +1893,13 @@ 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;
@@ -1918,6 +1918,12 @@ 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.
@@ -2401,7 +2407,7 @@ 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)
{
@@ -2743,16 +2749,18 @@ 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;
@@ -2812,7 +2820,7 @@ 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;
@@ -2849,6 +2857,7 @@ 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;
@@ -2856,23 +2865,29 @@ 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;
@@ -2886,14 +2901,19 @@ 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;
}
/**
--
2.30.2
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