[PATCH printk v2 29/38] printk: console_flush_on_panic: use srcu console list iterator

John Ogness
2022-10-19T15:07:18Z

With SRCU it is now safe to traverse the console list, even if
the console_trylock() failed. However, overwriting console->seq
when console_trylock() failed is still an issue.

Switch to SRCU iteration and document remaining issue with
console->seq.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/printk.c | 21 +++++++++++----------
1 file changed, 11 insertions(+), 10 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0c37ab208395..e478cb92e7ba 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2998,21 +2998,22 @@ void console_flush_on_panic(enum con_flush_mode mode)
console_may_schedule = 0;

if (mode == CONSOLE_REPLAY_ALL) {
- struct hlist_node *tmp;
struct console *c;
+ int cookie;
u64 seq;

seq = prb_first_valid_seq(prb);
- /*
- * This cannot use for_each_console() because it's not established
- * that the current context has console locked and neither there is
- * a guarantee that there is no concurrency in that case.
- *
- * Open code it for documentation purposes and pretend that
- * it works.
- */
- hlist_for_each_entry_safe(c, tmp, &console_list, node)
+
+ cookie = console_srcu_read_lock();
+ for_each_console_srcu(c) {
+ /*
+ * If the above console_trylock() failed, this is an
+ * unsynchronized assignment. But in that case, the
+ * kernel is in "hope and pray" mode anyway.
+ */
c->seq = seq;
+ }
+ console_srcu_read_unlock(cookie);
}
console_unlock();
}
--
2.30.2