[PATCH printk v2 27/38] printk: console_flush_all: use srcu console list iterator

John Ogness
2022-10-19T15:07:30Z

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>
---
kernel/printk/printk.c | 38 ++++++++++++++++++++++++++++----------
1 file changed, 28 insertions(+), 10 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7ff2fc75fc3b..c4d5d58b5977 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1851,7 +1851,7 @@ static void console_lock_spinning_enable(void)
*
* 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;

@@ -1870,6 +1870,12 @@ 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.
@@ -2353,7 +2359,7 @@ 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)
{
@@ -2695,8 +2701,8 @@ 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.
*
* Returns false if the given console has no next record to print, otherwise
* true.
@@ -2704,7 +2710,7 @@ static void __console_unlock(void)
* Requires the console_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;
@@ -2764,7 +2770,7 @@ 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;
@@ -2801,6 +2807,7 @@ 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;
@@ -2808,7 +2815,8 @@ 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))
@@ -2819,12 +2827,17 @@ static bool console_flush_all(bool do_cond_resched, u64 *next_seq, bool *handove
/* 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;

@@ -2838,14 +2851,19 @@ 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