2022-10-19 15:56:18

by John Ogness

Subject: [PATCH printk v2 32/38] printk: __pr_flush: use srcu console list iterator

Use srcu console list iteration for console list traversal.

Document why the console_lock is still necessary.

Signed-off-by: John Ogness <[email protected]>
---
kernel/printk/printk.c | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 809c43e596d8..2faa6e3e2fb8 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3454,6 +3454,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
struct console *c;
u64 last_diff = 0;
u64 printk_seq;
+ int cookie;
u64 diff;
u64 seq;

@@ -3464,9 +3465,15 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
for (;;) {
diff = 0;

+ /*
+ * Hold the console_lock to guarantee safe access to
+ * console->seq and to prevent changes to @console_suspended
+ * until all consoles have been processed.
+ */
console_lock();

- for_each_console(c) {
+ cookie = console_srcu_read_lock();
+ for_each_console_srcu(c) {
if (con && con != c)
continue;
if (!console_is_usable(c))
@@ -3475,6 +3482,7 @@ static bool __pr_flush(struct console *con, int timeout_ms, bool reset_on_progre
if (printk_seq < seq)
diff += seq - printk_seq;
}
+ console_srcu_read_unlock(cookie);

/*
* If consoles are suspended, it cannot be expected that they
--
2.30.2

2022-10-26 08:41:37

by Petr Mladek

Subject: Re: [PATCH printk v2 32/38] printk: __pr_flush: use srcu console list iterator

On Wed 2022-10-19 17:01:54, John Ogness wrote:
> Use srcu console list iteration for console list traversal.
>
> Document why the console_lock is still necessary.

When thinking about it. It might be worth explaining why the srcu
traversal is added even when console_lock is still needed.
It would make the life of code archaeologists easier.

I mean to add a note that it will keep the traversal safe
even after console_drivers list manipulation will not longer be
serialized using console_lock(). Or something like this.

It would be nice to add this to all other patches the introduced
srcu and kept console_lock.

> Signed-off-by: John Ogness <[email protected]>

Anyway, the change looks good. Feel free to use:

Reviewed-by: Petr Mladek <[email protected]>

Best Regards,
Petr