2022-10-19 15:11:42

by John Ogness

Subject: [PATCH printk v2 25/38] proc: consoles: document console_lock usage

The console_lock is held throughout the start/show/stop procedure
to print out device/driver information about all registered
consoles. Since the console_lock is being used for multiple reasons,
explicitly document these reasons. This will be useful when the
console_lock is split into fine-grained locking.

Signed-off-by: John Ogness <[email protected]>
---
fs/proc/consoles.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/fs/proc/consoles.c b/fs/proc/consoles.c
index cf2e0788f9c7..32512b477605 100644
--- a/fs/proc/consoles.c
+++ b/fs/proc/consoles.c
@@ -63,6 +63,14 @@ static void *c_start(struct seq_file *m, loff_t *pos)
struct console *con;
loff_t off = 0;

+ /*
+ * Stop console printing because the device() callback may
+ * assume the console is not within its write() callback.
+ *
+ * Hold the console_lock to guarantee safe traversal of the
+ * console list. SRCU cannot be used because there is no
+ * place to store the SRCU cookie.
+ */
console_lock();
for_each_console(con)
if (off++ == *pos)
--
2.30.2

2022-10-25 14:59:55

by Petr Mladek

Subject: Re: [PATCH printk v2 25/38] proc: consoles: document console_lock usage

On Wed 2022-10-19 17:01:47, John Ogness wrote:
> The console_lock is held throughout the start/show/stop procedure
> to print out device/driver information about all registered
> consoles. Since the console_lock is being used for multiple reasons,
> explicitly document these reasons. This will be useful when the
> console_lock is split into fine-grained locking.
>
> Signed-off-by: John Ogness <[email protected]>
> ---
> fs/proc/consoles.c | 8 ++++++++
> 1 file changed, 8 insertions(+)
>
> diff --git a/fs/proc/consoles.c b/fs/proc/consoles.c
> index cf2e0788f9c7..32512b477605 100644
> --- a/fs/proc/consoles.c
> +++ b/fs/proc/consoles.c
> @@ -63,6 +63,14 @@ static void *c_start(struct seq_file *m, loff_t *pos)
> struct console *con;
> loff_t off = 0;
>
> + /*
> + * Stop console printing because the device() callback may
> + * assume the console is not within its write() callback.

Like in previous patches, I would prefer to add more information
about this dependency. An example or if it is just to stay
on the safe side.

> + *
> + * Hold the console_lock to guarantee safe traversal of the
> + * console list. SRCU cannot be used because there is no
> + * place to store the SRCU cookie.

It might be possible to crate a custom struct for passing both
the next struct console and SRCU cookie. But it probably
is not worth it.

> + */
> console_lock();
> for_each_console(con)
> if (off++ == *pos)

Best Regards,
Petr