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printk: console_flush_all: use srcu console list iterator

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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
This commit is contained in:
John Ogness 2022-11-16 17:27:27 +01:06 committed by Petr Mladek
parent b8ef04be6e
commit fc956ae0de
1 changed files with 35 additions and 15 deletions
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50
kernel/printk/printk.c
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@ -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;
}
/**